An Update.. a twin, SA, oscillator - 5/8" bore x 3/4" stroke, at 60 psi (AIR) just about makes this generator go.... But the engine is well knackered - needs pistons that approach the same size as the bore, and big-ends that don't rattle loudly! - Maybe a new crank and pistons, with re-bored cylinders? - Time, and time... to do these things.
K2