The gauges are not multiplexed through the PCM so keeping them working shouldn't be too difficult.
The speedo works off a cable to the trans. The oil pressure and water temp have their own senders (go to the cluster from the black connector C110), the fuel sender has a slosh module but its in the cluster itself, plus the fuel sender is wired separately from the fuel pump (but in the same connector). The tach might be the only one that may or may not work properly; its a DG/Y wire. The wire goes to the cluster from the engine bay from connector C109 (gray by the brake booster).
The oil/water senders just need to be put on the engine, and your factory fuse panel needs to be kept intact and powered for the fuel level and volt meter. A tach signal needs to be found and hooked up to that DG/Y wire (although the signal being supplied to it may not be accurate for the factory gauge).
I have not confirmed any of this to work yet, just been messing with the factory wiring lately on my 90 LX as I have removed the factory PCM/engine wiring and will be using the Holley HP on it. From looking at the wiring diagrams it should be possible to keep most of it working without too much hassle.
90' LX________________2011 GT Premium, DD, manual
Previous 4 cyl.__________Brembo Package, Borla ATAK system
427W, in the works______JLT Intake, Tuned