VW tdi, Mercedes 300d or the Cummins 4bt are the most common swaps.
The 4BT is the cheapest to do parts wise. That engine is very heavy though and that sucker is big and tall. Still a very reliable engine, except for the injection pump those Bosch distributor pumps do go out and are expensive to rebuilt.
On top of that the BT engines love to leak oil all over the place
4bt is going to be your best bet out of a generator or truck. With the swap you should be getting roughly 25mpg lifted on 35s(atleast what I hear the norm is when doing it) but its a pain in the a** and usually can end up costing somewhere around 4-7gs to do
the rebuild on it is going to cost you a pretty penny as well as getting the right kind of pump...the price however does seem right if anything offer the guy $900 cash maybe he'll take it and make sure you find out EXACTLY what the engine came out of build date and all because a lot of those fuel pumps/parts were very specific to manufactured date
Yeah thats what they mostly come out off or generators I had a guy at a shop i work at who worked rebuilding these and he said he would give me one for $500 bucks and I freaking lost his phone number, I've been crossing my fingers ever since for him to come by the shop again lol.....I hope this helps however this guy specializes in swaps and maybe he'll be able to sell you an engine: http://jeffdanielsjeeps.com/
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