I live in a rent house that has an older Day & Night wall heater. A couple of years ago the fan motor went out and a repair man replaced it with a motor that is actually to large for the unit. I do not know if he tried to get one through the manufacturer, but the heater is very loud and according to him it is because of the size of that motor. The heater blows air very hard and goes off and comes on a lot when the thermostat is turned up for heat. The repair man says that is also because of the motor. My question is if I get the serial numbers of the heater will I be able to purchase the correct motor for this unit. I suspect it is 15 years old.

I don't even see wall heaters on their website anymore. If you leave a model number I will try to research it for you. I'm guessing that he didn't leave the old motor and that you have no idea what motor (make and part #) he put in.

It is very likely that the original motor for your unit is un-available and has been for many years. Not uncommon in the hvac field.