I just had a thought, the Crank Sensor
doesn't move, and it's mounted in the
oil pump where the pan gasket is, but
what if the half moon key sheared in the
crank, I have seen it before, and it was a
Zook motor too, that could change the
valve timing and affect the distributor
setting moving the timing
Was the timing retarded of advanced
when you found it was off

Retarded would be from the belt or the
crank cog slipping
Hope this helps
Wild