Not enough to make a difference on these engines.Wow it looks like some major pitting on the ends. I would say that needs to be looked at because that will change cylinder volume, thus air fuel ratio for those cylinders
I need to measure the valve guides and stems to see where they are at, but they appear to be good. I was amazed at how easily some carb & choke cleaner cleaned the black goo off of the valve stems. Normally it would be baked on and need to be wire brushed off.In stead of a broken oil ring, from looking at the valves, and being they are both intake, it looks like it's been pulling oil in from the top end. Is it possible the valve guides are worn so bad, it sucking oil down around the valve stem, on the intake stroke..?? Looks like it literally just baked some oil on the valve stem over a period of time, until they seized.
Have you ever had this engine torn down before? With the serious pitting on 1 & 4, I'm wondering if a PO somewhere back the line, put a sleeve kit in it, because it had gotten water in the cylinders, and was stuck. Even at that, I've pulled several heads off of tractors that have been stuck for a good many years, and have never saw pitting like that before. Just thinking again, somewhere back the line, someone did a quicky fix with a sleeve set, and to heck with the cylinder head, other than maybe lapping the valves in, then sold it.
Years ago(15-20) when we were getting ready to rebuild the original engine my dad made an engine trade with his brother(my uncle) who also had a SC with a non running engine. When we opened it up we found the pitting in the head, but checked for leaks and it checked out good. We put hardened seats in along with a new valve kit. The bottom end got a full sleeve kit, reground crank, and all new bearings. You can still see the cross hatch in the bores and never consumed oil. It had always run great until we fired it up last year after sitting for several months.