I know where a nearby 1Hz isnot to send you into the weeds, but it’s your cruiser absolutely do what you want. A diesel swap would be cool no doubt but definitely get the right engine as there are definitely ones that are more trouble than worth.
I fully understand originality and where that line is on an old modified vehicle. My BJ74 is technically “mostly original”.original drivetrain, axles, interior, snorkel, heck in Australia you could order them with the ARB from dealer. On the flip side it’s lifted, upgraded turbo, bigger tires, soft top kit, and a few other trinkets that only came on different models. Makes it more desirable for most people but fully original? No.
Find a diesel that someone is getting rid of for cheap. Look into the Mercedes OM617 or OM variations. It would be awesome with an OM617. There are some swaps here. As you also know @theglobb used a 4bd1t from an Isuzu, excellent engine with a crap ton of spare parts. You can find those old Isuzu cab overs all the time for cheap.
I’d love someday to get a rig like this. Right up your alley for a surf/beach rig. The only downside being your legs ARE the crumple zone
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On a recent trip to Japan I saw so many cool 4wd trucks- here's one that I particularly liked, especially it's name: