There is one other option, John. A computer fan like the one shown in the link here might work:
https://www.amazon.com/Nidec-dell-B35502-35-120x38mm-CN-0Y4574/dp/B00B87CTDA It is a 12v DC motor, it puts out a fair amount of air and it only uses about a third of an amp at full speed. You would just remove your fan and the motor and attach this on top of the heater core. The key is would you have enough room to fit in 1.5" (that being its depth)?
Somewhere in the attic I have one of these I'd be happy to send for the cost of postage and I may also have one that might be a bit thinner...can't remember that for certain but I did save some of these from a thermoelectric project I did about 20 years ago.
