1 hp is about the max that you will want to run on 120 power,
compressors are notorious for heavy current draw on startup. You might
skimp to get 1.5 hp but that would be a push.

Check to see what the HP the replacement motor is (or is that the one
you don't know about?).

The pump manufacturers rate the pumps at various HP depending on the
output pressure and the rpm. Example: speedair has one twin cylinder
pump with 1hp, 1.5hp, and 2 hp motors, the parts list shows the 3
different pulleys necessary to run each different motor.

You might want to look at the Granger catalog, try and match your pump
to a current production unit. You might have to pull a head off to get
the bore and stroke. Then check the spec tables in the catalog to get an
idea of what rpm you will need to run.

RB wrote:
Compressor is an older Champion roll-around.
Tank is about 10-15 gallons, compressor is a vee-twin, old U.S.-made
I just put a new pressure switch on it, only to find the motor is bad.
That motor was not original. I believe the original motor was 115V, HP
unknown.
What should I look for in a replacement motor?
I prefer to stay with 115V.

I tried a 1 HP motor that was wired for 115V, and it was not enough -
stalled and tripped the circuit breaker.
How do I determine what HP it needs?