Well getting a motorcycle engine is not as fun as upgrading the HF 6.5hp.. You will learn more as you build and mess around with things! That is what i like to do, i like to take apart and rebuild things..
I rather save the money i spend on a motorcycle engine and buy parts to upgrade my 6.5 engine
The motorbike engine will blow your mind if you think a modded clone is any good, even better, you can then mod the bike engine!
I completely understand you wanting to mod your clone, modding stuff is fun, bolt on parts aren't my style though (lol). Not much of the stuff I have is the way it left the factory. For example, I've got a 4HP Briggs lawn mower engine, I've modified it to run horizontal shaft, now I'm building a kick starter for it, once I've done that I've got an OHV head from a Honda that I've been modifying the block to fit, got to get the appropriate taps for the block and drill it, turn up the new lifters and fix the valves now. Once I've done that (and if it works) I'l pull the smog pump off my car, that should make a good supercharger for the Briggs

. Why am I doing all this? Because I can, I don't have any use for the engine, I don't have any vehicles I can put it on. It's just fun, you know?
I had an early 2000's xr100 motor lined up to be my powerplant but it began missing gears, backfiring, revving without throttling, and burning oil so I ditched it and got a 6.5

.
So it need a carby clean (or rejet, it was running lean), a new set of piston rings and some gearbox work? When I was at school we rebuilt one of those (1980s model though), **** it was quick for a 100, when you changed into 3rd it'd lift the front wheel a little

. That was a pretty awesome bike, especially considering it was a little ****ter of a bike that some one got for free.
Thanks
Hayden