Hi Kevin,
Well, putting in the 1KZ may not be an issue in itself if you have the engine mounts with it. But you also have to change the gearbox and possibly the propshafts (due to length mismatch)
If it is 4wd you might also have to take the gearbox for hi-low and engaging 4wd. It might be the same but I'm not sure.
The 1KZ typically comes with the R150 gearbox if it's a manual, and the 3L is usually coupled to a G52 transmission or other G transmission if it's a manual.
When it comes to the automatic gearboxes I'm not sure, they may share an automatic gearbox, but then you would still need a different bell housing.
You can check your gearbox code under the engine hood on the chassis number plate, it has more codes, also a TRM code.
If you just want a more powerful engine instead of the 3L, you might also consider switching to petrol. Like a 2RZ-FE or 3RZ-FE. Actually the 2RZ-E was only shipped in this van and that one had a carburator so I would not recommend it. But it means the 2RZ-FE also fits, it has 142hp. The 3RZ-FE has 150hp and is a bit taller, probably fits but not sure.
These RZ engines may need a different bellhousing, but other than that they will fit your gearbox.
If you don't use the camper everyday, just like me, it may be interesting to swap to petrol.
It also means less driving noise, but to be honest I only know the driving noise of the 2L engine (much like the 3L you have). 1KZ probably is a bit smoother, but still a petrol engine will be more quite.
Also note that the 1KZ is not a perfect engine. It is much more powerful than the 3L, but it may also suffer from a cracked head once in a while, and ECU's may sometimes file. However, all can be fixed
If you can find several 1KZ's, I would go with the latest model years, somewhere Toyota updated the engine to give the last cilinder better cooling.
Next to all that there is also the 2L-T or even 2L-TE engine. That last one is a more modern version of the 2L (2.4 litre) engine. Advantage is you get a smoother turbodiesel (like the 1KZ) but it will fit your gearbox and all without issues. The 2L-TE has been shipped in the Hiace / Super Custom, but only in Japan. Anyway, it fits.