Dave, I am wondering.... Have you been running the motor at less than full speed? The cooling on these is typically based on the motor running at full speed so the fan can move enough air to maintain somewhat reasonable cooling. If you find you are running the lathe slower then I would recommend either an increased reduction belt drive (so the motor can run faster whilst keeping the spindle speed lower) and/or a separate cooling fan for the motor.?
On my first heavily modified mini I added a cover to the rear of the motor and attached some light flexible hose (Hoover vac hose) that is attached to a small squirrel cage blower with a bit of filter media on the blower inlet. This keeps the motor cooler and also keeps swarf from being taken in the motor.
Just my thoughts, Cheers
ram50v8efi