Not directly. You could put something together using some parts from a 1862/1864 and the same era super garden tractors, but the conversion is not for mechanical novices.
I would make sure the breather reed valve and filter is clean and working properly. Worst case is there is excessive cylinder blow-by that is overloading the crankcase ventilation system.
Welcome from Elkland
Those transaxles are notorious for failure. No new parts are available. You will need to source a good used unit. Check our sponsors.
The M18 is an improvement even over the Series II KT17 it replaced. The only negatives I have with the M18 is the thirst for fuel, and the governor gear.
It would be helpful if you could provide the model number for the deck. There should be a tag riveted to the deck. There were at least 3 variations of the 42" deck that fit the 104. Most of the idler parts are still available new. The idler arm might be the only one you will need to source used.
Straight 30W oil has not been affected by the reduced zinc content, mostly due that it hasn't been recommended for use in cars or trucks in several decades, thus not in use with vehicles with catalytic converters.
I have converted a couple of battery ignition tractors over to the magneto ignition. I replaced the key switch with one from a 582 or 1811, then re-pinned the connector and ran wiring/ deleted wiring as needed. No issues with the conversions.
Any mechanical implement or vehicle needs to be exercised, much like the human body. Sitting idle for long periods is one of the worst things for them. Glad you were able to get it freed up.
In the event that the lever/linkage is seized, I would spray some Kroil or P-Blaster on it and see if it improves. If it is the valve body, they are very common if the tractor does not have the front hydraulic remotes.
The only things going for Rustoleum are price and ease of availability. The OEM sourced paints are better quality, but three times the cost and somewhat harder to source.
Do it once, do it right.
Sharp tractor. Technically, the 1711 is a 1985 model year tractor, but could have been produced towards the end of calendar year 1984. I too have one that has a Magnum 20 swapped into it that is my plow tractor.