It should be a standard HL airsoft unit, maybe with the ball and channel system for elevation, and should be easily replaceable. It was an RTR mato tank? TK22? What do you mean by "simulated cannon fire"? The sound and recoil when it fires? That's controlled by the micro switch so if the unit doesn't fire a round it won't trigger the sound effects. You might want to check for loose wires. If it were me I'd apply power directly to the airsoft motor to see if it's in the gun or in the wiring leading up to the gun. It's not a hard unit to swap out, and I think Erik even has a video but I'm not sure it shows a Sherman, which I believe uses the same elevation system as the Panthers. You might have to remove and re-glue the channel to the new unit, but that's about the only thing different from any other airsoft swap. Unless of course it's servo operated, and that's a different animal.