A new vaporizer is all it took to get my Caterpillar purring. My Clark that was about the same age as yours (I since sold it) had an unreliable mechanical regulator after the generator and it sometimes failed to charge the battery. When the battery was low the engine torque would go down until it eventually stalled. My best guess was a weak spark, as that was the only electrical part, but I didn't dig into it much. But run it while it's on a decent charger and if it runs great replace the battery and diagnose the generator/regulator.
It looks like it draws gas from the bottle, which can freeze up from extended use in cool weather. But, it can run off of a regular propane tank too if you have the adapter. You won't have a vaporizer, but you will have essentially a "propane carburetor" which is just a regulator with a negative pressure output. They run about $60 for either type so it's a cheap tune up.
Last time I bought a forklift that old it didn't even have hydraulic valves or a brake cylinder included. I made an offer for the not running forklift, bypassed the hydraulic pump and replaced the battery and drove off with it.
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