For the best in "go-anywhere" engines I'd go with a 200Tdi, second would be a 300Tdi. There may be "better" overall choices but the parts needed for a 200 or 300 conversion are all off-the-shelf since they were both factory engines for the D1.
The 200 needs no electrics to run and the 300 only needs one wire for the fuel shutoff (and that can be done without in a pinch but you'll have to stall the engine to shut it down).
Tom Rowe
Four wheel drive allows you to get stuck
in places even more inaccessible.
62 88 reg
67 NADA x2
74 Air Portable - The Antichrist (tag 6A666)
95 D1 - R380
95 D90 - R380
97 D1 - ZF