Old, ATF could very well do the same as seafoam but I took this smoker that I used fewer than fiver hours a year, had every intention of overhauling, and is no longer a smoker. After a coupla hours usage I had to replace number four spark plug to stop the miss. I started using it more about three years ago and put a new can of seafoam in with an oil change. It has stopped smoking except a very little on start up. I worked over ten hours fitting ground last spring and again last fall. Since I worked it hard two years in a row without smoke I have not had to change plugs. I bought the (sorry red) Farmall C in 2002. The first oil change was like transmission grease. The oil now runs clean. The fellow I bought the tractor from said it has set for several years. Dirty wore out oil and setting apparently caused the oil rings to stick in and did not expand out tight against the cylinder walls or it would not have stopped smoking.
It was smoking when used in the picture above.
Kinda hard work for this size tractor. Made her pull down and snort in heavey hay.
Don't give up on a engine that seems sound and smokes after setting for a long period.