Hi Tiger,
The score marks on your cilinder/piston come most likely from the metal debris of the big end. That's thanks to the almost horizontal position of the cilinder.
Perhaps you ran the engine very hot at some point in time? or too low oil level? Or just asking too much from the engine. It's hard to say anything with 100% certainty.
But.... when you put the engine together again use rags, not paper. Buy degreaser by the gallon and use a big bucket to clean the engine halfs. Wash the parts twice, rinse with water and dry with an airgun.
When you start up for the first time just let the engine run for a couple of minutes, than lean it over to the left side, almost flat, take out the two oil plugs, clean and mount again. Do this a couple of times, running the engine or ride a km.
You'll notice that the screen filter will catch a lot of stuff; silicon gasket, fibres, etc. If it is a lot It will clog the screenfilter. KTM even gave an official warning to their dealers about it because aparantly quit a few engines seized after being overhauled...
If you fail to do this your engine will sound fine at first but after a couple of miles/kms it might seize for a few seconds. That was one of the rockerarms seizing on its shaft and getting loose again.... the rockerarms are at the end of the oil circuitry and will suffer from insuffiecient oil supply first.
happy wrenching,
