Finished rebuilding the compressor thanks to MTR's guidance. Replaced all valves (including one broken one), all gaskets, crankshaft seal, and new pressure gauge. Performed another test to calculate the CFM expecting better numbers with new valves and gaskets.
Got these numbers: 100 psi kick in pressure, 122 psi kick out pressure, with an elapsed time of 30 seconds. That gives me 13.2 cfm as my compressor output! Well I'm happy I got higher numbers, but 13.2 cfm sounds too good to be true. The new gauge I installed has a range of 0-300 psi and I'm sure I'm reading the pressures right. In fact I did the test twice. I might get another gauge with a lower range to see if I get different numbers. Or should I just be happy my cfm is a lot higher than before the rebuild?

And last question: Are the kick in and kick out pressures I'm getting about right for this compressor? My pressure switch has a sticker on it that says its factory set. How are those numbers determined?