My idea of a life span on these is" lets see if I can blow it up!!!!"
So far no luck..... well I did have a 48cc or 50cc what ever small size it was supposed to be and I'll add bought from bgf it lasted long enough to start, seized in about 1 minute from the trash that was in the crankcase. So if you buy from him you will want to take it completely apart, clean out the cigarette butts, ashes, metal filing, finger nail clippings etc. you might get a good engine out of the deal on the other hand you might not.
Don't bother with asking him for a warranty there isn't any.I've bought more engines from him than anyone else and they are a crap shoot. Not that he is a no good guy just you never know what going to land on your door step from him.
I bought 1 kings it was the first and still going strong and it runs very smooth no vibs.
Dax is another seller I've bought from his engines are very, very, nice I'd rate his as the best of the few that I have bought, plus he's very good about parts etc.
But I do have parts now.