Is the rev counter fitted connected to the orange wire from the ECU? Or is it an inductive tachometer connected with a strap round the HT lead?

If it is the former, it could be that where it was tapped into the orange wire it has cause a bad or broken connection. If the ECU isn't reading rpm correctly, or the dash for that matter, then running problems are quite possible.

As bonjo mentioned, disconnect the aftermarket tacho and check all the wiring. See if you can get a reading on the dash as a starter...

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