I ain't no expert! I am just lucky
As far as finding things for it goes; Find the ID plate and get the Model and SPEC #. That is what you will need to be certain your getting the correct anything for it. Miller actually has manuals to most of these machines, call with those numbers and they can ship one out to you for cheap. Engine parts takes a little stubbornness at the NAPA but they can be located cause they were used in tractors, cars etc. The generator/electronic side is much tougher...fortunately there is not much to go wrong in there. A lot of it is easy for a good electronics guy to make a copy of as well. I have some links to graveyards and such buried somewhere on this PC
This is a straight six, single barrel carb, mechanical governor yea? Assuming it is running smooth and clean.
Strother check the brush set at the end of the generator first...make sure you got some left. Run the machine on high and stick a voltmeter in your 120v outlets and see what you get. If it is running smooth but you are not getting 120v or close to it, then yea she is not turning fast enough...if you are getting 120v then you have some issue in the welding circuit. If you determine your NOT getting enough speed...I would check ignition timing with a timing light FIRST. It does not have to be off much on that old ford to lose a lot of performance. Next probable cause would be the governor. They are a real pain to get set just right, probably impossible without the manual procedure. If ya decide to mess with the governor shoot me a picture of it...I may have the procedure I can copy you.
Good luck
let us know what you figure out