The 90 is an oddball. You can only use a 90 ECU, unless you want to swap a couple pins as outlined in the vfaq at
www.vfaq.com. Naturally it will be MUCH easier to find ECUs for 91-94, especially if you are looking for an EPROM ecu. A friend of mine has been looking for a 90 EPROM for a while now with no luck. IF you just want a regular one, then any ECU from a 90 AWD that matches your tranny (auto or manual) will work, and non-eprom ECUs should only be ~50 bucks (EPROM will be ~250)...