Tapered shaft. Long road ahead.
Yep, first step is to tear down the engine and take the crankshaft to a machine shop and have it turned and a keyway cut.
I'm guessing it's from a generator, so probably a fixed throttle. Gotta work around that too.
Unfortunately not a lot of people here are running diesel, so mechanical advice will be slim as far as the engine is concerned.
Besides that, find a rolling frame on Craigslist, buy a clutch, throw some fat tires on it, and stick it all together.