These are my thought, which might not be worth much!!
OK, I doubt that insulation would be an issue at low voltage so the principal limitation on safe working voltage will be maximum current carrying capability and hence wiring temperature.
So, I expect you could safely hit that motor with two or more times over voltage for a short period of time, maybe very short.
I expect that if you could put a light load on the motor and increase the voltage from zero it would start and get faster with increasing voltage, at some point the rate of increase of RPM with voltage increase will slow and I suggest that would be close to maximum safe working voltage.
Safe working voltage also varies with load as a light load would allow the motor to turn faster which would mean more back EMF generated which in turn limits the current and hence the heating of the windings. So it is all pretty complicated if you want to get technical.
For me, 12 volts is probably a good guess for that sort of motor and if it does your job at that voltage well and good.