swoop223 has a good point- use good (not aftermarket) igniton wires & spark plugs ( I have good luck with A/C or Autolite plugs & NAPA make your own wires) The SMH stuff never seems to hold up.

If the ignition checks out, I'd then check the fuel system. Does the engine only run rough at idle? When was the last time the Impco regulator was changed? Is it a model J? What type of propane shut off valve does it have- vacuum VFF-30?

I have found that usually when the impco systems go bad they either are hard to start & won't accelerate when cold ( buildup of tar) or they begin to run rich when warm (primary port of regulator starts to leak) or VFF-30 vacuum line starts to feed engine LPG.

To check the VFF-30- pull the vacuum line & cover the vacuum port. Engine should run a few minutes before stalling- listen to engine to see if engine smooths out. If engine runs same reattach vacuum lline, start engine & shut off propane bottle. Listen to see if engine smooths out right before it stalls. If it does the regulator is bad.

Hope this helps