A lot of try and error
You just has to check different ways of getting that special shape you want. The engine started as a cylinder, and then its just a lot of loop divide and extrude and pulling in vertexes. The reference for the engine was a bit small, so I had a hard time seeing the details. I'm on the body now. Added wires and the engine reference I used. And one of the pictures I use as reference for the bike.