Thanks a lot for the replies.

Yep, the only reason for running at 48 volts was for the extra speed, as it was I was only just reaching the required speed at 48V. If the motor hadn't failed I was thinking about trying another 12V to see if I could get the start temperatures down.

Another thing I didn't mention is that the motor has to sit between the pumps, a huge mass of piping and the load compressor air intake so the available diameter is a tiny bit larger than three inches. Which was another reason I chose the Magmotor.

I honestly don't know what the required torque is, the Magmotor didn't seem to have any trouble spinning the engine up so I guess I could get away with less.

If I read the Wiki page on torque and horsepower right I think I can calculate the steady-state HP as: 70A * 48V = 3360W, 3360W / 746 = 4.5HP.
And then the Torque as: 4.5HP/(10000RPM/5252) = 2.36 Pound-Feet

I don't know if that calculation is right but that is only 453.12 Inch-Ounces so I guess gearing it wouldn't be a problem if I could design some sort of vertical gearbox that fits in a 3 diameter space.