There are a few parts that are getting very hard to find - notably the exhaust manifolds. They are different than the ones from the S-10, and simply are not available either through the Isuzu channels are through the aftermarket world. You are going to have to go to the junkyard world for those. Again, unfortunately, a lot of junkyards are not keeping 1st gen Isuzus in the yard due the (relative) lack of demand for the parts.

While some people have made or had made headers, to my knowledge there are no Isuzu-specific headers available for the 2.8/3.1/3.4.

The transmissions are not interchangeable between the 2.6 and the 2.8 et al.