OK- as far as the ignition- spark should be bright blue & jump at least 1/4" gap at the spark plug end.

You are correct that the LPG system consists of a lock-off valve, regulator & mixer.

The lock-off valve (either electric or vacuum activated) is self explanatory. If electric- make sure it flows fuel when given 12 Volts. These can gum up & stick in the off position- flowing no fuel. If vacuum- make sure flows fuel when given a vacuum source. This style goes bad when the o-ring which seals the activation pin goes bad, feeding LPG through the vacuum feed line, making the air/fuel ratio overly rich,no matter how you adjust the fuel mixture.

The regulator- takes liquid fuel at a maximum of 312 lbs, lowers the pressure to around 6 lbs, allows the LPG to "boil" (which converts the fuel from a liquid to a gas), and also acts as a secondary fuel shut-off- allowing fuel to flow only when a vacuum is sensed through the fuel delivery line. These can gum up, not allowing fuel to flow when the unit is cold. The symptom of this is the engine will start (usually a hard start), & the engine will only idle. If you give the gas pedal any movement, the engine will starve for fuel & stall. If the engine is allowed to run long enough, this symptom will go away & the engine will run normally. As L1ftmech stated, they can also fail to regulate the fuel, sending much higher fuel pressure to the mixer than the mixer can handle. Symptoms of this are icing of regulator, usually will only run at higher RPMS (will flood & stall at idle), & extreme burnt smell of LPG in the exhaust.

The mixer- takes in vapor fuel, mixes this fuel with incoming air, & feeds this mixture to the engine. It also acts as the third fuel shut-off- allowing fuel to flow only when a vacuum is sensed through the throat of the mixer. These fail because the main diaphragm becomes stuck to the fuel outlet in the throat of the mixer. Engine backfires can also rupture the body of the diaphragm.

As you can see- engine vacuum is the key to this system working- no fuel will flow without it.

Symptoms of this type of fuel system are usually pretty straight forward. If you find an issue with any part of the system I would recommend that the entire system be re-done. As EdT stated, a engine tune up would be a good idea too.