Before I check the valves, I would look at the drivers side cylinder and rings. Oil
is keep separate in
the crankcase. Within the cylinder only fuel, air and exhaust are ever present. The
Valve's stem does
extend from the crankcase so checking the valves is not totlally out of the question.
I'd have to say
there is a more likely chance of cylinder wearing than the higher pressure
blowthrough zone within the valve
chambers. Fuel passes the screens in the carbs reducing debris intrusion although
is still possible, now however,
if an air filter is compromised, cylinder damage on some level is more likely. SO
keep them filters clean and your engines
will have a longer running life. Also change the oil as often as you can, every 5
hours of run time if the environmental conditions
are anywhere near harse.
Good luck with getting it back in order. Oftentimes damage on this level reduces
the engine's value by 90% or more. replacing 1 worn part
while not replacing another doesn't make a good fit or match due to the un-even wear
of part's hours of runtime. just keep your fingers crossed when investigating it
cylinder walls and chomp at cyclinder rings ultimately resulting in oil throughout
the remainder of the engine.