Ask yourself if you want a two cycle or 4cycle engine. Very popular engines are the Honda, Mitsubishi, Robin-Suburu, and Tecumsa. Would you perfer a chain drive, roller-drive (simpilist), or belt drive. Is weight an issue. A motor between your legs is usually heaver than the rear mounted. Do you want you engine to come w brackets or can you make them? Does your bike have rear suspension re; mountain bike?