JieM,
Was there a lot of carbon build up on the valve stem? You saw in the picture of mine after the cylinder failure, the exhaust valves in particular had a LOT of carbon on them. Mine also had that groove, although it was not as pronounced as yours. I have waited about 45 days for valves to come into stock, although that was with my supplier, they may be available somewhere else.
I am doing the intake conversion to negative crank pressure, and then I am going to run it on 30:1 ratio as it wont need as much oil going past the rings into the crank, I am hoping this will reduce the amount of carbon build up. I may also consider moving to Stihl oil instead of redline, as some at my field have reported less carbon build up on their engines (not all Saito engines).
Not saying anyone should follow my lead, but I am doing some experimenting.
These engines are not built to last a lifetime like some others are.