Piston problems
Given that the problem is oil fouling related and not compression i would be looking at the third (oil control) ring very carefully.
Is the ring installed up the right way?
Is the spring installed correctly behind the ring?
Did the mech score the ring groove or side of the piston when removing/installing the ring? (Oil can bypass the ring down the score mark)
Are the end and side clearances for the ring correct, it is not uncommon for new rings to need the ends adjusted on lycomings.
Is the ring the correct oversize for the cylinder?