You do not say how you are priming the carburetor?
Some units were equipped with an optional low oil pressure-fuel shut off switch that would shut off power to the LPG lock off valve when the oil pressure was low, but allowed power to the lock-off during the cranking part of the start cycle.IF it is installed, it will be a 3 wire sender mounted on a 'tee" fitting in the same hole in the block (end of the oil galley) that the single wire sender (for the dash light) is also and normally attached to. You may have one of those wires broken, or the engine is so worn out the oil pressure may not be high enough during cranking. Are you getting good battery voltage to the lock off when you are trying to start? Do you have fuel flow through the lock-off while cranking the starter?

This is a dual fuel, (from back in the days when I considered dual fuel to be a mean joke played by factory design engineers and sales types [who loved the way it looked in sales literature], on the forklift tech in the field, [who was supposed to make it work the way the end user expected]) so I would consider that the timing may be about 5 degrees off from where it should be as an LPG machine, and you may not have as much vacuum as you expect, partially due to how the gas carburetor and lpg mixer mount, and that this motor NEEDS to turn pretty fast to draw enough fuel to start up, so if your cold crank amps in the battery are not as hot as you can get (?what does the battery drop to in volts while cranking or under load test?) you can have these sorts of problems

It seems a shame that you have already replaced all the LPG parts, as I often suggest to owners of this age dual fuel machines that only use 1 type of fuel to convert the machine back into a single fuel machine. SMH has the kit to do a conversion to only LPG for a very reasonable cost, and the kit includes the whole ball of wax, including hoses and vacuum plugs, linkage, etc...