There was a big article in Toyota Trails a while ago about diesel swaps, check the TLCA website (www.tlca.og) and see what you can find. Also do a few searches on this forum and check out the tech section and see what you can find. Keep us posted if you decide to go with the conversion, good luck.
The best isuzu diesel for a landcruiser would be the 3.9lt 6cyl if you can find one.Landrover sold a few hundred of them fitted to Countys and army spec vehicles in Australia in the mid 80s and they are still popular .At the time of their release they were considered equal to any toyota diesel.They are speed limited to 3000rpm.I think there is a smaller version about 3.6lt as well.
I wouldn't touch anything that said izusu on the valve cover. There junk, enough said. I had two of them, don't ask! Think power stroke if you want diesel. In this day of age your better off swapping in a fuel injected big block, with the price of diesel nowadays you'd be better off.
I would be interested to know what model and piece of equipment is running an Isuzu diesel engine in a CAT...
Caterpillar purchased part of MHI (Mitsubishi heavy industries) back in the early 80's, and uses some of their smaller engines in some units, they also use smaller Perkins in some pieces, and also use Hatz in some smaller paving compactors.
If you are aware of a piece of Caterpillar equipment that is using an Isuzu, please let me know.