I may have to stand corrected here. I have worked on many different two cycle engines. One I know of relied upon the oil on it's plain main bearings for a seal, another relied upon a separate seal over the caged needle bearing main bearings. I have to concede that I am not sure if the seal on a sealed bearings races would be an effective crankcase seal on a two cycle engine. The seal on a bearing my just protect the bearing from contamination but may not be an effective pressure seal. You know what they say about the word assume.

Bill