RE: Black piston?
By the looks of it I'd think that maybe that engine could use new bearings anyway...so...I would completely dis-assemble it and soak it in the crock pot. Scrub it good...and provided the sleeve/piston fit was worth while, put new bearings in and fly it.
After you get all the parts clean, they'll be easier to inspect for signs of excessive / unusual wear.
I have one TT .46 w/ over 20 gallons through it (yep...20 gallons ) and it doesn't look anything like that...just minimal carbon build up, and v. little gunk on the outside... (Powermaster 10% sport fuel, 18% oil which is a castor/synth blend )