What Jerry has said above echoes what I've read in various places - I have even heard that the two engines share the same block casting. My guess is that the 4JG2 would drop right in.

This is, however, all anecdotal evidence, unfortunately -- now you'll get to pioneer a diesel conversion, too, Paul! (Although I really think the process will be basically identical to what I've done with the 4JB1).

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com