I would guess you have already looked through the FAQ's and found the section on failed speedometers:

https://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm#Speedometer_Failures

There is a hot link there to Overseas Speedometers (https://www.speedometer.com/)

Several years ago I posted in this forum about my contact with Overseas Speedometers with an inquiry about repairing a failed speedometer in a 1991 740 with leaking/failed capacitors. The nice/timely reply from them said that yes they had repaired them in the past but no longer did. Apparently they had had an employee that accepted the challenge of removing the old capacitors and cleaning up the board that had been damaged due to the leakage and sometimes had good results. Because of the amount of time involved and mixed results at the repair attempt they no longer accepted those speedometers for repair. I looked for my post but was unable to find it even with a Google advanced search. you might contact them to see if they can provide any leads for a repair.

The other approach is to source a 1993-94 940 cluster and swap it into your 1991. The speedometer will work fine without any alterations. However the gas gauge won't register.

The work around on the gas gauge is either swap out the sending unit in the gas tank or alter the circuitry in the cluster. I worked with another brickboarder on the necessary steps and then wrote it up. I think it might be in the FAQ's as sort of an addendum. I also must have it on my harddrive somewhere but off hand I'm not sure. If the cluster work around is of interest to you let me know and I will organize a search.

EDIT:
The search was successful. Here are two links that might be helpful:

https://www.brickboard.com/FAQ/700-900/ElectricalInstrumentClusterInterchangeability.htm

https://www.brickboard.com/FAQ/700-900/ElectricalInstrumentClusterInterchangeability.htm#FuelGaugeClock

Randy
--
Any twenty minute job is just a broken bolt away from a three day ordeal