Will your bike be a kit or a DIY'er type?
I am geared toward doing all DIY stuff and like to see what people come up with. Of course there are also kit bikes that have a bit of mods that I say catch my eye also.
I had built my prior motor bike back in the same 70's era as you did with the help of my dad which modded my drill and bolt through frame approach. Sort of weakens the tube. It was better when he made a brazed platform to be able to move engine fwd and aft so to adjust a direct drive v-belt to washing machine pulley on rear wheel. It was a wood sandwich connect to spokes on I think 24inch wheel frame.
Now I want a versatile design with many gear ratios. Can't ride the streets (no pedals on it and over 2hp), but eventually to get OHV sticker for CA state and Fed BLM motor cycle parks and the private ones too.
I have a jackshaft that I am just now going to adapt a bit to allow a second 10inch like washing machine pulley that goes to a belt clutch I expect to get if I think it can work. It will give about 25 to 1 where as now I have 8.3 to one ratio. With one of the jackshaft smaller pulley changed I'll vary from parade speed to hill climb to well maybe Lands Speed on salt flats if I can think how to get in contact and have time to get with that done.
The situation is that the 10 inch pulley will be right close to the 2.3 inch belt clutch and the amount of belt that rides on the smaller pulley due to the angle it has when it reaches the belt clutch pulley has me cause for concern that there might not be enough friction.
I have a belt clutch I'd like to mod to be able to use, but it must be 1/4 not 5/8 as the shaft it is sort of stuck on is a Briggs 5S which I just rather keep for something not needing to have much power.
I was thinking it would probably not be very easy as going from a smaller shaft to larger, you just shim.
Anyway the 70's Era Aluminum Briggs has 3hp I revamped and is about the same weight as the cast iron 5S but 1/2 again as much hp, so that is what I'm using.
I'm not sure but I think the 5S would be about 2hp. Both engines had the valves and seats cut to restore the angles, so they have their compression like new.
The 5S was a trash engine with stuck valve I fixed, the newer one was $5 at SJ Flea Mkt from my time tinkering 30 years ago but now have it on a trash pulled 60's Era JC Penny Foremost 26inch frame that is nearing completion.
You may see my post on the two above pages