Thermal protection for an electric motor can be easily and cheaply
added, but nothing can protect against a hot spot in cotton covered
wiring.  A hot spot can be caused by an original defect or from
mechanical or electrical damage over the years.  The problem is that
cotton covered wiring, when saturated with oil, can catch on fire
from a local hot spot.

That said, the best way to add thermal protection is with a thermostat
made for an electric water heater.  These can be bought for about $15.
They can certainly handle the current and are made to act as switches,
operating thousands of times before failure, unlike other thermal
protection devices.  You can also adjust the temperature setting very
easily.  Just set it very low and slowly bring it up until you are
comfortable with the setting.

Some spots on the motor are more indicative of the internal temperature
so it is a good idea to use a non-contact infrared (IR) thermometer to
find the best place to mount the thermostat.  Tie wraps can be used to
temporarily attach the thermostat.  Once you locate the perfect place
for it use something more permanent like stainless steel tie straps
and/or some epoxy.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/