You know what, that reminds me. I was working at an auto parts place once when these guys came in with a Mazda B2200 pickup that had a 3-cylinder tractor engine in it. They said it got between 75-90 MPG, but it took about 5 minutes to get it up to 55.
It may be "illegal" on the books, but people over here in Oregon pretty much do what they want with a vehicle. The DMV doesn't pop your hood to register it. Heck, if it's already registered and being driven, I dare say you could do the swap and drive it until it dies and nobody would ever know the difference. I suppose it depends on where you live.
The main problem with putting in a diesel we never got here is that you can't buy parts locally for it. Ever waited 3 weeks for an overseas order for one oil filter?