If you have the right size piston and rings in a freshly bored cylinder then your issue is most likely a valve. Valves are pretty cheap. I've replaced (and lapped in) them in every rebuild I have done so far. You could have the perfect ring gap and the highest compression piston imaginable and if the valves aren't sealing....it's crapola.
It's sounds insulting but it's been done MANY times...you sure the right size piston and rings are in there? If so, pull that head and rework it, then a new base and head gasket.
I was born and raised on Venus & I may be here a while.....