Ah, the B&S engine is balanced for the rotational load of the 21 blade at a constant (relatively) demand. But can I get some use from it is my question, sir.
You could probably get it to run OK if you used a heavy cast iron pulley on it. The vertical makes it a little hard to use though. You can’t run them on there side... the oil distribution won’t work, they don’t have an oil pump.
If you were able to get it to run smoothly and connected a car alternator with the diodes removed, you’d have a 3 phase generator (put 12v on the rotor and you can get 5 or 6 KW at 100v pretty easily.
This 3 phase alternator will run another car alternator (also with the diodes taken out) as a brush-less motor. You could run anything with that motor.
Or, you could just use an unmodified car alternator, put it on a small cart and pull it behind you E-Bike. (you’d have to adjust the rotor winding voltage to get the desired DC voltage for your E-Bike.)