Motor HP will give some clues as to its performance. However, if the motor was changed around with "something that fit" rather than the right match we may be off. So, motor sheave diameter, pump flywheel diameter, and cylinder bore and stroke would be helpful. You can measure all of these, but honestly I wouldn't even try to pull the valves as they may not seal as good when you put them back down and its unlikely you will ever find parts for it. It looks to be a low HP unit based on the motor. If you time its fill up and tell us what final pressure was we can also tell you what its current performance is. One of the dead give aways on that compressors age is the secondary base. They haven't used those since at least the 1960's. The formula for CFM is
CFM = (tank gallons) / 7.48 * pressure / 14.7 * 60 / time(seconds)
That looks like a 60 gallon tank to me and the nameplate suggests the same.
Basically on something like this you run it until she blows and replace the pump. It's nice that it's so old and still works, but a lot of places won't bother with them anymore because they've been out of production for many decades.