Looking for some info.
me and my son's are building a cafe racer with my k100. 
I'm looking for a rear shock that's an inch or 2 longer than the standard RS model. Something from another bike maybe to try and keep cost down
thanks.

You could try a shock off a K1100 or other later model K100 with paralever rear-end. They measure 385mm. A std K100 is about 350mm. So you'd get about 1½"
The problem with this is you will push the driveshaft down a few more degrees and and place more stress on the uni-joint because the driveshaft is already angled down to start with and you'd be pushing the driveshaft past its normal range of movement.
You'd also be changing the geometry of the frame slightly and placing more weight forward. The front end would probably become a bit more responsive.
Also, I think the paralever shock is a little stiffer due to it's bottom mount being a little further back placing it at a less upright angle.

Just my thought. Cheers

Does anyone know if the R100gs uni joint is different to ours, as they have a greater angle and behave just fine mile after mile. Gaz what about the R80gs, is it a greater angle than the k's you own?
There must be a safe range to operate in.

Uni joint range 0° to 16° for continuous running with 24° as an occasional maximum.
At 0° the rollers dont spin and so leave marks across the inner and over 16° the rollers will spin fast enough to generate excessive heat leading to the eventual destruction of the bearing surface and ultimate failure.

If its height you are after I mounted my seat a little higher on the frame.

I have seen what looks like a second hope welded on top of the frme to give the same result

Years ago I got a set of Koni shocks for a xs850 yammy, they were a a bit longer maybe inch and half, made it sit up higher, but was scary trying to go around corners it just wanted to go straight, put a set of xs1100 shocks on and was perfect, just my experience so maybe Ks are different. Gary

I'm with Garyk100.
Apart from the angle of the driveshaft Uni joints (Rick G),lifting the back end will change the steering rake angle. I don't have enough expertise to say how much of an issue it would be, but without serious expert input I'd be reluctant.

Sort-of relevant experience was with a Mini Cooper S. For a short time, the springs which pull the rear end down on the hydrolastics were not fitted. The extra ride height at the rear changed the steering geometry so much that it was difficult to go around any corner without tyre squealing.

Also on Uni joints, someone who raced minis used to have the suspension height set so he driveshafts were horizontal. He attributed his success at the time to the competitors who had their minis lowered as much as possible losing energy from inner uni joints running permanently angled, in otherwise very comparable cars.

Rear ride height on mini was personal experience, drive shaft angle hearsay, but direct from the person who did it and very knowledgable about minis in both road and racing settings.

Bill