I can say I would like to see it done as with the motor you have just to see if it would work, but I do have to agree it is more than I think you would want.
By the way when I am done with a motor bike project I am looking to use an engine like the one you mention to do a hovercraft. With ducting one engine can work the thing. But it is a little better with one Vertical shaft engine for the lift and a separate horizontal shaft for the propulsion. I dreamed of it long ago and am going to see with time given to do it also.
Discarding that anyway, about do the easier way, I have seen the same 6 to 7.5 hp called vertical shaft engines used to make not just a lawn mower blade spin, but on an axis 90 degrees opposed spin the front wheels of the mower itself. The self propelled mower. How it is done is quite simple. Just having two pulleys and then put a half twist in the belt to accommodate the axis change. A belt is not like a chain so it can do this without a lot of additional wear. It has to have enough length so it does not rub at a cross.
The other place I saw this done was on a replication as best I can tell of a Radio Flyer Wagon with the half twist belt. It went not fast at all because it was at the Burning Man Event (too many people to have fast vehicles moving) and the rules are 5mph and under.
The wagon did not have any other than the usual steering and so it would be very dangerous if not numb skull stupid to try to configure it to go fast. Not to say a motor bike should be this slow, but 20 mph is quite alright I’ve found and for off highway vehicle woods areas, hill climbing is nice to be able to do! With this wagon I saw I'm sure there also was gearing down with some method I did not see, but it had to be some. Even though the tires are like 1/3 to 1/4 th the diameter of the motor bike tires gearing down was still necessary to be able to go slow enough.
For a motor bike with the engine you have being vertical shaft engine mounted, I still would like it where where it would be in the most balanced position on the frame inside the 3 bars that make a triangle. Maybe it would fit, but most likely it would involve modifying by cutting and adding and re-welding back tubes and having a platform for the engine. Adjustment for when the belt stretches built in also very important and I would have to engine able to slide forward and aft.
I’m finishing a motor bike like I made before long ago, but it just is a bit more refined with what I have done. There is a short chain to a jack shaft that allows further gearing down as well as allowing the engine to be centered by weight on the frame and the shaft sticks out a bit to the left. Then on a pulley on the same live jack shaft axle the smallest pulley I could use to go to a larger pulley attached to the spokes on the rear wheel with a v-belt (gearing down). Also note I'm using a Link Belt. It allows for lengthening and shortening the belt. This is something that never existed as far as I know back in the 70’s.
The half twist on other than what I have mentioned and seen works would be what you might devise to do.
I have in my mind what it could look like, but quite a number of jack shafts and pulleys with one of the sets of pulleys having a belt off to the side of the frame parallel to the bike with a half twist. Then it runs a jack shaft as I have mentioned above. To get to a position where a pulley could turn and have enough length for a half twist (as too short a length cannot work) an extremely short belt would go sideways and connect to a small jack shaft right from the engine. There could be a centrifugal belt clutch mounted right on the engine shaft too, so you could idle and not move and you would not have to shut off the engine and restart it again when moving.
Yes Rube Goldberg would probably approve and Milton Bradley might come looking as Mouse Trap is probably at least fifty years old now and maybe time to do another Rube Goldberg project. I mean at least I found that Shlitz Beer has arrived at BevMo and touts a Classic 60’s Formula made by a small brewery under license. I just love it when on 70’s show Red Foreman grabs a can of beer that was from an ice bucket that Michael had shook every can in it before unknowingly Red Foreman takes a can to open. Sprays all foam from the can then and he shouts Shlitz. Why mess with a good thing except that it can work for a sitcom! Maybe Michael did a good thing not even knowing it….. the writers anyway for the 70’s show