I have a similar project taking place. The only 4 cylinder motorcycle engine I can recall is a honda "goldwing"? I believe. If I am correct our projects are very Alike for I have a 500cc 50HP liquid cooled handa silverwing motor I amm putting in a G9. If it is a honda It shouldnt be as big a problem as you would think. What you need is a 4 stroke golf cart, due to the f/r differential. What I am doing is mounting the motor and taking the shaft that turns the wheel and shortening the length up quite a bit, then making a coupling that connects the shaft of the rear to the shaft of the bike.
Hope this helped and if my engine will make my cart scream I would love to know what yours would do.