either clean that spark plug in some petrol, or carefully with a wire brush etc or pay $4 for a new one. if it gets fouled again, there may b esomething wrong with that cylinder.
if there was oil gettting into the intake, it would be on all the plugs. either it is, and the other cylinders are happily burning it away, but no.6 is a weak spark etc. or the cylinder with the bad plug has bad rings or another problem. If your mechanic wants to rebuild it it may be cheaper to just buy a second hand engine rb20s are cheap as chips.
as a quick check, swap the coil and plug to other cylinders and see if the problem moves too. how does it run, obviously stalling but does it rev smoothly or sound like a tractor