These engines do not require valve seals on either valve they do not smoke if they are removed. A valve seal on the exhaust valve is certainly not required, the manufacturers only put them on the intake because the oil can be drawn down the guide on the intake stroke, all pressure on the exhaust stroke is pushing out of the cylinder therefore oil will not be ingested. There is a very small likelyhood that oil is entering the cylinder by way of the valves, it is usually the head gasket not sealing at the edges next to the oil galley or push rod tunnel. The rings and cylinder have to be in really poor condition to causes excessive smoking, which if this is a fairly new engine is unlikely. I have taken engines off wood splitters and pumps that visually you can see the cylinder wear and feel valve guide slop and they hardly show any smoking or plug fouling, and they just kept churning away.