A regular compression gauge will not work properly on rotary engines. Mazda has a special compression tester that gives a readout for all three combustion chambers per rotor.
From the symptoms you describe, it sounds as if the engine needs a rebuild because of low compression, which in turn would be simply because the seals are flat worn out. 140,000 miles isn't bad at all if this is the original engine, though I've heard of Mazda RX-7s pushing 300,000 miles on the original engine because the owner exercised diligence, proper care, and went the extra mile on maintenance.
If you're really interested in the car, start looking for rebuilt engines from reputable suppliers (tip: not the dealer). Without experience you should not try to rebuild it yourself, but if you have experience it would be fairly easy.
After a proper rebuild or exchange, look at running a small amount of premix or MMO in the fuel along with a good oil in the crankcase per the owner's manual. The new seals, especially the apex seals, will probably last a lot longer. Also, during a rebuild get the radiator and oil cooler flushed: cheap insurance.