The PC fan thing, I don't think that will work. You can buy high performance motor fans that will fit right on your motor heatsink. Not sure what sources you would have for them in your area, but here, we can get them from Amain, Amazon, ebay, etc.
As for the driveshaft, you could try creating...
Welcome to the forum. If you add the fan outside the chassis cover, I think will eventually destroy the fan, but it may work. But then you'll have to remove the fan to remove the cover. You would need a filter on the fan if you did that, at which point, you might as well just use the fan under...