I intuitively suspect that boosting the voltage would be safer with a wound field motor than with the PM motors used in some later ETs. I'm pretty sure that wound field motors usually withstand overheating better than PM motors.
*nod* PM motors basically have a "fixed" field in the permanent magnets, and thus you will run the motors.... much faster and under a 25% field weakening value at higher voltages. Expect a lot more heat, a lot higher current draw, and dropped magnets (which can be fixed with the JB Weld and jigs; I have not had a repaired motor drop another magnet)
C