if you want to try, you can open the ECU, note the capacitor values and orientation, there is a black line on them indicating which side is negative.
I'd ignore all other caps , just focus on the electrolitic ones, the ones in tiny aluminum cans. these are really cheap parts.

you can clip the capacitor just above the board and bend the new leads around a pin so they look like tiny springs, and insert them over the little wires you have left, then crimp and solder there. normlly they are pushed through the PCB and soldered from the backsde but that may require breaking the heat sink putty and replacing it. my thoughts are if you do this from the topside it may be easier. if you clip the capacitor can with snips about 1/8" above the surface of the board there should be enough left there above the circuit board to solder to.

No guarantee that will improve things but the caps can fail , they are cheap and I think if you carefully replace those it may help things. they can be checked in place using a ESR meter
https://en.wikipedia.org/wiki/ESR_meter

that may help you not need to remove the ECU from it's aluminum heat conducting case. I'm assuming they are similar to other ECU's.. If you like you may be able to get a reconditioned one from Rock auto.

The rev limiter is a new one one on me, and I'm not familiar with this rex/regina system.

The original notes indicated that the issue began with the replacemt of the transistor or power stage or what you like to call it, so trying to "fix the ECU" may be just leading the OP off in a different direction. Difficult to guess if there could be more than one issue involved in the cause of the problem.

I think if I was ready to order a rebuilt ICU you may try and if if it helps great, if not successful and things do not go well, then proceed with the order and eat that cost then. It may save a few bucks if you are successful .

possibly opening it could affect the core charge?