The original Duo-Art motor is not as efficient as a modern motor
and will run somewhat hotter than a similarly sized modern one.
Yes, they can get quite warm after three hours of continuous
playing.  If the motor was rewound with modern enameled magnet
wire it can get very hot without failure and is much safer than
with the original cotton covered wire.  Other things might be
damaged by the heat but the motor windings will survive.

If the motor is getting almost too hot to touch then something
is probably wrong.  The most likely candidate on a Duo-Art that has
a good pump and tight stack is an improperly adjusted spill valve.
If the valve is too tight, it will increase the load on the pump
and motor and can cause severe overheating.

Best regards,
Spencer Chase