I am in the final stages of an upgrade to the front braking system of my 1983 K100. The system is a little hybrid but I think it will be a big improvement on original. I purchased 2 new Brembo Gold P4 30/34 callipers and machined an aluminium billet adaptor plate to fit onto the original fork legs. I found what others had discovered, the standard front brake master cylinder (12 or 13mm) has too small diameter to exert the necessary pressure and after looking at the complexities of fitting a non-BMW larger diameter master cylinder abandoned that idea in favour of fitting a K1100 or R1100R 20mm diameter Magura M/C. These take the K100 right side switch gear. If you use non-BMW switchgear you are in for a major rewiring job. 
The 20mm master cylinder housing has a different throttle cable mechanism. The earlier K100 throttle cable will not work with it. I purchased used K1100RS and RT throttle cables but the longer of the 2 cables was still too short. I botched shortening the outer black plastic/steel cover on the cable, cutting several strands of cable wire while "carefully" using a Dremel-type tool. What to do? The key parts required to home make a suitable replacement cable are the plastic end mouldings on the K1100 cable - the top bent plastic moulding that locks into the throttle/front brake master cylinder mechanism and the bottom plastic flange/tip that fits into the cable bracket on the throttle body. On my metal lathe I machined from brass a copy of the plastic flange and at the thickest outer part drilled and taped a M3 thread through to the internal hole the cable fits into. I fitted a M3 grub screw and after tightening have a strong and secure bottom cable flange. The flange will not come loose, and, because it can easily be removed, then refitted, it is possible to easily adjust throttle cable length, or, build a new cable. I looked online at throttle cable repair/making kits and was horrified at their price (I'm also horrified at the price of new cables and listed second hand cables). I had an as new Renault tailgate cable on hand. It was long enough to make a replacement cable - the wire was of suitable size and the cable outer has a plastic liner allowing the steel cable to move smoothly without lubrication. After machining up two cable end BRASS nipples to K1100 size I soldered them on and have refitted my new custom cable - works perfectly. 
I did an online search for "motorcycle throttle cable kits" and "Motorcycle Bowden cables" and was horrified at the prices being asked. I've found an alternative source for affordable suitable stainless steel cable. Bicycle gear change cable is correct specification and available at a fraction of the cost. Hope this helps and inspires - not for everyone.

It's handy gear that bicycle stuff I used some to make new cables for the lift mechanism on a ford Fairlane some years ago and saved around $100 on the cheapest repair kit I could find and AFAIK it's still going.

I finally had to replace a cable on my Brick ( 26 yo and 165,000 k's)which turned into a comedy of errors. The throttle started to occasionally stick and after confirming the throttle bodies were fine the gearing was fine and there was no interference from the grip it was down to the cable. It was found to have two frayed strands at full extension where it goes through the upper metal ferrule. As I was removing the cable the adjuster fell off. I have a clean garage floor but it was nowhere to be found. I removed the tank the airbox and the belly pan, no adjuster to be found I searched the floor again still no adjuster. I rang the BM shop and he had one $30.00 Au and I also got a new spare cable $40.00 Au. Chris said that if I ever find the old one I can bring it back for $30.00 store credit. When I slid the tank back I also tore the overflow hose off which to my surprise I found I had a metre in stock. Next problem when I fired the bike up I had a fuel leak at the return hose. I had previously had a problem with it kinking so I fitted a Unicorn Unicoil. When I removed the hose it was found to have delaminated and split. I had cut off the bit that was kinking when I fitted the Unicoil but in hindsight I should of cut it back even further. As a temporary fix I cut it back another 4" and added a heavier duty piece of hose with a joiner. All up 5 hours of fun. Some where in space is my adjuster is orbiting the Sun with all the other lost parts.
Regards Martin.

We call those 'Five minute jobs that take all day', they do happen.

Another way round the brake issue is to bore out the K100 master cylinder to 16mm and make a piston to suit - I copied the piston in my 1100s and used those seals.

Not my idea, I bought a Brembo P4 kit for my K100 way back when, and it came with an exchange master cylinder bored out to 15mm. This was never big enough for my liking, so I bored it out further myself.

PS when you finally come across your cable adjuster, it'll be hanging out with a load of little bits and pieces you don't recognise - they're mine.

Somewhere else in space there is also a monstrous ball of Barbie doll clothes most of which belong to my two daughters and now grand daughters. Funnily enough my sons Evel Knievel never once had his clothes removed.
Regards a perplexed Martin.

Bowden cable can command stupid-price and the shorter the length, the pricier it is. I buy it in 30m coils, at which point it becomes relatively cheap per metre. I also have two small plastic drawers filled with various cable fittings salvaged from dead cables over the years and another small drawer filled with pre-made brass nipples. However, BMW being BMW (and as we all know), their cable nipples are a different size to everyone else, so I also have a small cardboard cylinder with three lengths of brass rod in it just for LFB...

If ever in need, lacking the skill to fab your own cables, and in the US or Canada, www.motionpro.com in southern Calithumpia have a custom cable programme. Tho' they prolly call it a program. Good stuff, not terribly expensive, and built to last.