I have a Gx160 and I don't remember seeing any seal and if it did, it's been running a long time without one just fine. I have changed the valve springs alot too from #18's, #26's, #55's.
I have pulled a couple of clones apart that didn't have em either but they were modified. The only engine I have that has one is a Pred 212 and that also the only brand new motor I have ever bought.