I have a 3020G I picked up for cheap but mine is working good so far, after a good tuneup and some TLC.     You can get a owners/service manual from Miller.   Just contact them and they can probably dig you one up and ship it for 15-20$.    Mine has the SCRs in the control board, yours might.....if you lose those I do not think you can get any output ( they are easy to get and replace)....but I am not a welder tech either.      The manuals have some good info like how to set the governor and troubleshoot the generator + a good parts list.      Parts are hard to find but I have seen a few shops that say they still service these and can get parts......If I figure out where I stashed that info on those shops I will pass it on to you.

I am pretty sure that this one should idle at 1000RPM and weld at 1800RPM.   If you get surging/hunting at weld speed it is more then likely a mis-adjusted governor (you need a manual because it is tricky to adjust), probably not a weak solenoid.  I will have to check but I think the ignition timing is -10 BTDC.  Mine was way early when I got it.  If you still got a data plate on the motor it should be there if you can still read it.

BTW. Depending on the year of that Ford   a lot of parts like from a 93 Ford ranger will work on that motor....like main seals, some bearings etc.

The machine is probably worth fixing if the motor is strong.   I run some 7018 verts with mine, some 6010 etc. and it is smooth and buttery.  I like it.