Once you decide that OMC power plant has served its useful life and beyond reasonable repair, I’d recommend an engine out of a Goldwing if it would fit (horizontally opposed 6 cynlider for modern ones 1500 or 1832 cc and will have a output for a drive shaft.). At that point you may need a 4 pt harness and a grab rail for passengers and while you are at it if you want to turn some heads either straight exhaust (would sound like a finely tuned F1 car with straight pipes) or weld a washer into the Pipe if you really want to get noticed
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