A lisp is nothing. Stuttering will depend on the serverity, I'd imagine. I knew an artist who had a serious stuttering problem. It would take him several seconds to get a group of words out.
Althoguh I've never met a pilot with a stutter, judging from the non-pilot I knew, I would say it shouldn't be a too much of a problem. I can't recall any medical reg. requiring perfect speech. Can't be any worse than the southern drawls encountered, or the French and Chinese pilots out of JFK. I feel for those controllers. Some of the communications between certain foreign carriers are downright scary.