I've built/converted a couple of K75s from monolever to paralever final drives. Why? Because it pretty much eliminates the "jacking effect" that lifts the monolever rear end under heavy acceleration (as much as you get out of a K75 engine anyhow) and makes it handle better on high speed turns. Although installing a 3" wide 3-spoke rear wheel and running radials probably makes a bigger handling difference I do both and it makes a K75 handle oh so much better.
JMHO, but I wouldn't bother doing this upgrade to a K100. I think that for the time, money and effort involved you'd be much better off just selling your 2V K100 and buying a K100RS4V or a K1100 because from the factory those bikes have:
1 - 10 more hp
2 - much better front brakes
3 - the ability to run radials on the stock rims
Anyhow, here's what you need to do:
Transmission: The transmission needs to either be from a K100RS4V or a K1100 so that it has mounting tangs for the front of the paralever strut. Alternatively, you could find a later K bike transmission that has the tangs and get them drilled for the paralever strut but I find it easiest just to use a paralever transmission from a 4V K bike.
The only difference in transmission gearing between K75s and K100s/K1100s is in fifth gear. A K75 has a 1.67 gear ratio for fifth gear while the four cylinder K bikes have a slightly "taller" 1.61 ratio for fifth gear. The gear ratios for gears 1-4 are the same. This is about a 3% difference in fifth gear and not very noticeable.
The transmission for any classic K bike will "bolt right up" to any other classic K bike as all of the transmissions use, aside from the paralever strut tangs, the same housing.
Swing Arm: You'll need a 4V K bike swing arm and the rubber boot that connects it to the paralever final drive.
Paralever strut: Obviously you'll need one of these. The later ones are better IMO because they are stamped metal and not solid - which makes the bike a wee bit lighter.
Drive shaft: You'll need both the front and rear halves of the 4V drive shaft. The good side of this is that the splines on these are much less subject to wear under load since they don't move back and forth under load like the monolever ones do. (Although the U-joints seem to be more prone to failure under heavy loading - probably not as much of an issue on a K75 given the lower hp.)
Peg Plates: Since the paralever swing arm has a "bulge" in it to accommodate the pivot bearings, stock K75/K100 peg plates won't work. There are several approaches to dealing with this issue. One is to shim the rear peg plate bolts with washers so that it tilts out a bit and avoids contact with the swing arm "bulge." The other approach is to use either K100RS4V or K1100 peg plates. The K1100 peg plates require an additional mounting bracket that resides under the bottom of the lower battery mounting bracket.
K100RS4V peg plates:
K1100 peg plates:
Shock: The shock on 4V paralever K bikes is about an inch longer so you'll need to replace that too.
Final drive: The "shortest" final drive that you can get is a 32:11(K1100LT) so that's what I recommend to maximize torque. (K1100LTs also came with a 33:12(2.75) gear ratio.) The gear ratio is stamped into the flat metal surface around the breather cap on the top of the final drive.
