The power connectors are prone to corrosion near the fan itself. Try a little vaseline and a good scrape.
Failing that, either the motor (test with 12v) or the resistor pack (usually green, about the size of a packet of Swan Vestas).
With total failure, I'd suspect the first two.
SPI and MPI units don't appear to be fully interchangeable -- at least, not to a quick glance.