Hey, if you go to 'gasbike' under 4stroke parts, they have a tapered shaft clutch that goes with the 'primary' KC vale could give you good info. Even bicyclengines prolly has the same deal. The 5/8 shaft is for your own clutch, while the 78 is referring to the diameter of the centrifugal clutch the goes with there 4g belt drive primary
On the 5/8" long shaft engine you have to mount your own clutch AND it's bell to the actual engine shaft and then a transfer case around it.
The clutch bell rides on a bushing, not any bearings, and needs a lot of service.
The clutch attached short shaft engine (142F-1G) is all set to bolt a 4G full bearing supported clutch bell and output shaft belt drive transfer case right to it.
It's the best combo I've found for trouble free operation good performance and you can get creative with it...
For example, if you can't fit a 4-stroke 4-G in the bike frame with a SBP shift kit, just add a second set of mounting holes in the 4G and rotate it.
I do the same thing but the other way for my long shaft 10G's.
It has to go down so the long shaft clears the oil filler.
I had some longer output shafts for a 20T/100TFW 4G made, and 'press replace' the new longer shaft to the transfer case to put the left hand drive to the right hand side.
Granted, these are not things Joe Casual Builder can do well even if they had the right tools, but that's not my point Sam, I am just showing off hehe ;-}
I recommend going with the HS 142F (or 144F if you can find one) -1G clutch attached short shaft engine and 4G transfer case.
There are several versions of the 4G, I use a special version for my 10G's, but just go with what B-E has, it all the right parts for most people and one of the few places you can get the real thing.