The diesel is an interesting option but pricey and difficult; you'll probably have to import an engine from Japan (new) or Canada (used) along with the right ECU and everything else you need. And then good luck finding replacement parts quickly for an engine that was never marketed in this country.

Still a diesel would be an Isuzu solution and (probably) dramatically increase your fuel economy. But I would also consider a swap to an Ecotec or V8 depending on price.