Is this a Continental engine?
Such an older machine some things to consider if you haven't already: vacuum gauge reading should be 18-22 inches at idle, compression test, cylinder leak-down test, ignition system, LPG components.
Compression should be above 165 psi (fully charged battery, all plugs removed, throttle held wide open).
Tune up: points, plugs, condenser, cap, rotor, wires. Spark plugs can be gapped at.025" - 0.28" since propane is more difficult to ignite. If the distributor has a vacuum advance test it and the breaker plate movement.
LPG system: rebuild or replace.
Valve adjustment.
Timing. Set at part throttle while stalling the tilt function to put a load on the engine. Set for highest rpm then check for easy starting, no pinging under a load, good power, etc. After setting the timing adjust the fuel mixture.
If the engine is tired (low compression) you can try one- or two-step hotter spark plugs to see if that helps.
Good luck.