RE: How to determine if piston and liner need to be replaced?
Those deep scores were not from anyting from within the engine, and they're not a product of your fuel.
It's obvious from the baked on castor that you've flown it way past any warranty responsibility on the part of OS. It's also enough buildup that it needs to be removed. There was a really good article in one of the magazines about how to restore an engine that's seen a lot of castor fuel. Can't remember which magazine, but it's probably the AMA one since it's usually the only one with articles beyond ARF reviews. You basically have to manually remove the buildup.
Castor is magic but it's not without it's cost. You're seeing the major one. It builds up and will start to cause heating problems. Today's fuels that are a mix of oils are really, really good fuels. They have enough castor for the overheating protection, but not enough to cause buildup at too fast a rate. Those of us who flew a lot in the old days would have to cleanup an engine at least once a season. Back then, the piston/sleeves weren't too expensive too.
Tower has the 46AX for $106. They have the piston/sleeve for $70. I wonder if it'd be priced like in the old days if more modelers today built their own kits and maintenanced their own engines.
Those scores look bad, but what really matters is how many are there and far more important, how good the piston/cylinder fit is. You won't know that until you've got it back together and running.
The OS.35S used to last through 2 or 3 cleanups before one would need a new piston/cylinder. And then it'd need a rod too. And next time it needed the piston/cylinder, the shaft bearing would have opened up enough that you'd not bother.
I use commercial 10% with a blend of oil, and I've got a 46AX that's seen 3 hard years. The piston is about as dark as yours, but there isn't the varnish buildup. Think about changing to fuel with a blend of oils. I'm one that used to run nothing but 25% castor with 7% nitro and if we'd had this modern oil back then, I would have jumped on the commercial oil-blend stuff in a heartbeat.