Problem: The temp guage stays at the bottom.

If I leave the car on idle till the fan kicks in the needle is still in the same position.

If the tempsensor was broken then the fan wouldn't start right?

Can it be the temp guage that's broken? It worked fine before the engine swap

The radiator fan uses a temp sensor on the rad itself, the guage uses one on the block.

Assuming this is SPi.

Earth the gauge wire to the block and check for full scale deflection of the gauge.

Cheers

GC