I would strongly suggest to get a manual for you model.
No, I don't think so. A 90Ci engine is equal to 1474.835 cc.
If they didn't have engines, they would just be very heavy bicycles.
no bigger then 50cc under a provisional license
Of course! With enough fabrication and the right transmission you can put just about any engine into a motorcycle. Several people have put air-cooled VW engines into motorcycles. Some with BMW's use...
Well, that depends on what motorcycle you are talking about. The are thousands of different motorcycles, each having a different ratio.