I am planning to do an almost non-stop 36 hours ride from Athens - Greece to Lisbon - Portugal which is around 3.600km and back a few days latter. My 1989 K75 has 220.000kms but has been meticusly serviced by the local official BMW Dealer for the last 20 years that has been in my hands. All things consider, which spares would be a good idea to carry with me just in case? (Fuel pump is the first thing that comes to my mind...)

The only time I've used spares in 20+ years of ownership was two clutch cables on other mates bricks. I do carry heaps of tools and spare parts in the belief that if you have them you won't need them. Minimum spark plugs, clutch cable, clutch lever, brake lever, throttle cable, tie wire, cable ties, insulation tape, self-amalgamating silicone tape and BMW tool kit. A well maintained and cared for Brick is your best insurance. I'm up to 180,000 K's and it has never let me down. Piccies of what I take now with a spare gear lever and a Motopressor.
Regards Martin.

Martin has it pretty well covered.  Only thing I add to his collection is a Hall Effect sensor plate and an air compressor.

One additional thing I do is have a packed "rescue box" at home.  I keep it with a collection of hard to get spares for almost anything that can fail and cause the bike to not run.  It has things like an ignition coil, ignition control unit, relays, turn signal module, temperature sensor and cooling system parts.  I leave it with my wife when I leave so all she has to do is address it to wherever I am stuck.  It is in a Priority Mail shipping box so it will get to me in three days or less.  With it, I can fix pretty much anything short of a blown engine in a hotel parking lot.

These bikes are super reliable, and if you do a good prep session before the trip the bike will get you there and back in good form.  I do all my own work, so I know the bike and it's condition intimately.  This is probably the most important part of riding them long distances with confidence.

I'll have to redo my spares piccies I do carry a hall sensor and a Motopressor tyre pump. I've also updated my tyre repair kit to a Motopressor all in one tool. 
Regards Martin.

Thank you both for your replies. Very impressive and well thought feedback! Would you trust an aftermarket fuel pump or the OEM is the way to go? My local dealer charges a bit more than 400 euros...

As long as it isn't one of the dirt cheap ChiCom pumps, I have no problem with aftermarket fuel pumps.  I have no idea what you actually get for the extra $300+ BMW demands for their fuel pump over a good aftermarket pump.

Gryph my $36.00 Au Chinese pump has been running since July 2017 and hasn't missed a beat. It has been tested on some pretty hard conditions long fast trips in 40C plus days. I did carry my OEM pump in the tail as backup which has since been replaced by another Aliexpress pump. See link http://www.motobrick.com/index.php?
Regards Martin.

Martin, the ones I'm thinking of are in the sub $20 price range.  If it was my bike I would look in the $40-50 range.  No need to spend more than that.

I've never seen them that cheap, are they a direct copy? I still love my AliExpress rear master cylinder I've even got two spares in stock, but not needed for me. However the stock goes up and down.
Regards Martin.

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I seem to recall seeing some in the $20 range a few years ago when I was shopping for my spare.  Shipped from China.  I know about the master cylinders, but they are much simpler than a fuel pump.  The motor adds a higher level of complexity and material cost to the fuel pump.   And I have not had good experiences with cheap Chicom electric motors.

Point-Seven-five wrote: I have no idea what you actually get for the extra $300+ BMW demands for their fuel pump over a good aftermarket pump.

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Stavros wrote:Thank you both for your replies. Very impressive and well thought feedback! Would you trust an aftermarket fuel pump or the OEM is the way to go? My local dealer charges a bit more than 400 euros...

Go to tills.de for a fuel pump. I don't think anyone buys an original.

92KK 84WW Olaf wrote:
Go to tills.de for a fuel pump. I don't think anyone buys an original.

And it's 1/3 the price of one from MotorWorks.

Stavros wrote:Thank you!

I just bought form them a Hall Sensor to have with me just in case...Yesterday I bought a new MAHLE pump from ebay for 74 euros and a 2nd hand ignition switch from kpartsholland.com. I have already ordered the clutch and gas cables from the local dealer. (I do hope that nothing from the above will be needed...at least not alongside the highway, in the south of France in an August Saturday night...)

Keep a spare clutch nipple on the bike. If the cable breaks at the lever the nipple vanishes. . . ,. . . you are sitting there with the spare cable and no nipple. It does not come with the cable.

Rest assured that if you have a spare with you, that part will almost never fail.

Point-Seven-five wrote:Rest assured that if you have a spare with you, that part will almost never fail.

definitely!
must be Murphy's law, I guess

bad boy wrote:

Point-Seven-five wrote:Rest assured that if you have a spare with you, that part will almost never fail.

definitely!
must be Murphy's law, I guess

Over here, Murphy's law would decree the part that fails is the one that has no spare. My emergency kit has been reduced to a tire pump, tire plugs, and a button accordion—to save weight.

I believe it's a corollary to Murphy's law that stipulates carrying a spare prevents failure of said part on the vehicle carrying said spare.

Another corollary that applies to marine situations is that no matter where one is on a vessel, when something of value or necessity is dropped it will always land in the water.

I always bring breakdown recovery insurance with me, so far not needed and I have done a 3500 km trip from Ireland to Germany and back and various other long European trips.

Failures I had were back brake switch (going in to Rome) and Rear Brake Reservoir cracking, both on my 1987 bike.

Ringfad wrote:I always bring breakdown recovery insurance with me, so far not needed and I have done a 3500 km trip from Ireland to Germany and back and various other long European trips.

In the U.S. the gold standard for breakdown insurance is the AAA.  Their RV Gold card covers lodging and towing up to 200 miles.  It will also cover anyone traveling with you. 

The bonus is that AAA contractors respond a lot faster than suppliers to other insurance companies.  This is important when you are out in the middle of nowhere and it's getting dark.