I am a Guzzi rider, I am also a diesel mecchanic. For some time the idea of a diesel motorcycle intrigued me but information is hard to come by. I invite you to contribute! I will post What I find over the next few weeks.
Sure you can. Yanmar clones are common on RE's. The 4sp is preferable, but I've heard of people doing it with 5 speeds successfully. They mount fine and are
For Enfields such as the bullet the raccomended engine is the Lombardini 15ld 350. Less important than the brand is if the pto is compatible with the gears