94 HO into 90 XJ swap

I have picked up a '90 XJ thats been fitted with a 94 HO engine. Having some issues with very poor gas mileage, so am wondering if the '90 computer is the culprit here, or what other changes might be needed? O2 sensor was done under the recall in '94. All polution stuff is stock '90. Have checked plugs, map sensor, etc.

how much stuff did you swap with the HO engine? If you did the HO injectors with the 90 computer, you could have some problems.

and the fuel pressure is and issue also.

Don't know how much stuff was changed with the HO engine, as it was done by a previous (unknown) owner.
What is the difference in the injectors? I thought they were all the same, how do I tell if they are '90 or '94?
Can I still retain the '90 computer, and what else has to be kept from the old setup?