Miller Econo Twin HF

I just picked up a used Miller Econo Twin HF (not econo tig) welder yesterday. It is my first tig and I was looking for something inexpensive to cut me teeth on. I paid $150 for it with a full 80cf full argon bottle and cart, but no torch. The serial number tells me it was made in 1979.
I have some questions about this I was hoping to get info on.
I bought it without being able to power it up. I measured the transformers to make sure nothing was burned out. I took the cover off and apart from a kluge repair on one of the cables leading to the electrode terminal, nothing looked fried. I want to clean it up before I fire it up. Some of the connection lugs look a little corroded. Does it make sense to take each one apart and clean the connection, or should I just leave it alone.
This unit varies the output current with a linking coil that slides in and out of the main transformer using a hand crank. The foot pedal only turns the HF on and off. Can anyone tell me how a modern transformer based welder varies its output current electrically? I'm pretty handy with electronics and was wondering if it would be a practical retro-fit for this machine.
Thanks Mike M.

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