Lots of old machines around here, and none of them have AC. I work the swampy woods where mosquitoes and black flies are a constant irritation, and thought a small window ac would be nice so I can keep the doors closed. Run it off an inverter, then I can rip owner tools too and to need a generator. But the old alternators are usually 45 amp, not enough to run an inverter unless the engine is a high rpm, an deven then with older alternators their days might be numbered from prolonged heavy draw. So what about getting a high curren talternator installed? Is it better to rewind, or go all new, maybe get a modern voltage regulator is there is better charging at idle?
If you're going on the super cheap, a 1.5kw generator for 120 bucks and a small window AC (540 watts) $120 and you'd be stylin. Small generator would run more than a few hours at 500 watts on a tank of fuel. Most machines have an air intake filter for outside air, you'd have to close off the cab vent and you could supply the factory blowers with cool bug free air without cutting glass or holes. just need a little outside sheet metal work and a bungee or six.
If you have room leave the alternator alone and add a belt driven generator head. Only problem would be the ac might burn up at idle? or maybe it would keep chugging along at low capacity? For an alternator, get a truck version. Don't mess with the original alternator.