Compression is pretty low, but consistent for all but two cylinders. Bring the suspect cylinder to TDC and add compressed air through the spark plug hole. Listen at the carb, tailpipe, oil fill and radiater. The sound of air leaking out the carb is a bad intake valve, tailpipe - exhaust valve, breather - rings, bubbles in radiator - cracked block/head or bad head gasket (unlikely in your scenario). Repeat for other low cylinder. You could also do a good cylinder for comparison on sounds. A poor-mans leak-down test. A pair of heads should fix it for a while if the rings are OK.