1KZ-TE will fit in both 1st/2nd gens. If you're swapping out a 22re engine, my hunch is it might fit the engine mounts (can anyone confirm this?) I took a look at my bro-in-law's '94 JDM hilux surf and the 1kz-te will easily fit any toyota pickup/4runner that has a 4-banger.
Almost any of the Toyota diesel engines will fit in a 1990 Hilux. I am sure the weight rating is up to it. I regularly put 1HD-FT engines with tranny and TC in a Nissan 850 kg rated truck. Make sure you tie them up really well and put some blocking under them so that they don't tip over while driving home. Also be prepared to wash out the bed when you take the truck back to the owner, some guys get all funny about oil and stuff.