Re: Removing pedals off motorised bike and adding pull start
There's a seal just like that on the other side of the crank under the bevel gear, it might be worthwhile to replace that one as well if it's in similar condition. If you got your engine out or apart this is a good time to change the crank seals.
The thing about running it with a bad seal or both seals bad is that it'll have that air leak where it can suck in air bypassing the carb and it can run the engine lean or at the least, make the engine very difficult to tune. It's just cheap insurance to replace while its apart and these seals only cost a few dollars each.