My speedodometer stopped working and my engine light is on.

check to see if actuator is moving to lock position

got a loose wire connector . check by jiggling all connectors in harness. good luck. dont give up prolly the grounding circuit.

Find the oil sensor on the engine unplug it and ground it the light should come on if so the system is working
if the light works after you test it do a continuity test on the sensor replace if necessary

I assume the motor controller PCB would be the problem.

you answered your own question

Once an engine seizes, it must be replaced or overhauled. Full stop.

Some models have a reverse gear lock on them which will make them stop when the engine light is 'on'. If the engine used was one of these and yours didn't have the reverse gear safety on it that could be your problem. A Polaris dealer will be able to tell you from the engine number what year it is. If it is the same year and model, it could be the timing adjustment. If you validate the timing adjustment, then its the carburetor. Rebuild it.

You might need a new thermostat, the temp switch may also have to be replaced.

Modual probably gone

Check fuses and wiring connections also the fan could have burnt out