You do have an oil seal on only the intake valve..
With the valve in the head, just off the seat, is there any back and forth play?
If so, (probably not), that would help with oil getting past the seal and burning oil.
You could do a "Leak down test" to ascertain the health of the engine however
I think you correct with the diag of worn piston rings. After cleaning the head,
flip it over, spark plug installed and fill with say seafoam-Let it sit. If the valves are leaking
the fluid will pass the valve(s) and into the runners.
Re the staining. As long as you don't have any scrapes (usually vertical), I
would just hone the cylinder before the new new rings are installed.
I have a similar engine from 2005 (18HP Briggs) which needed a head and piston (dropped a valve)
A new head, honed block and new piston/rings, runs like a top (no damage to the block).
Honed and new piston installed:
