I've done the engine upgrade.
Went from a R65GS 20KW to the R65LS 37KW.
Changed the heads, carbs, pistons, camshaft and the top air filter piece (one big one small pipe instead of two small pipes, don't forget that one!)
BIG difference in power, a bit more low end, end where the 20kw version stops the 37KW keeps going in the high revs.
I still have the short final drive (31/9) sometimes i think its faster then a R80.
You will still lose it on te straight above 140km/h but who cares.
I like the small engine of the R65 more than i like the bit extra power of the R80.
I do think you need to do the complete swap, all the 20kw to 37kw parts are a big difference, i don't think it will work well if you only do some parts.
My top speed with short final drive and 17" rear wheel (supermotard
) was 168km/h on the gps (over 180km/h on the speedo)
So against all the NO sayers, i'll tell you, Do it if you can get ALL parts for cheap.