if you have the original harness there's not that much work to get it going, just hook up the engine side harness and jump the wires that go in the cab (there should be about 4 wires that need to be connected, +12 switched, ignition, crank and maybe a ground or 2) - you can even connect them straight to the battery to start it. At this point you shouldn't care much for the gauges, you can get a set of mechanical ones for something like $20 with a mounting pod (oil pressure, water temperature and voltage) - just to get it moving. I hate to say it but we did warn you to get a YJ engine or at least the full YJ harness to make your life easier.
Don't give up now, you're really close - it's really not that big of a deal. I don't have a manual for the XJ renix otherwise i would have looked to tell you what wires to connect where but i think you do so map out what needs to be connected at a minimum.