If it ran fine before tearing it apart, and now runs like crap after you put it back together........highly likely it's from something you did during the stator replacement. Go back and double-check your work to make sure you didn't leave a wire or connection loose. Look at everything you touched during the work, or anything close to where you were working as I've knocked wires loose accidentally that were close to the part I was working on. Sure, a dirty air filter can cause issues but how would it suddenly get dirty with the machine sitting in the garage having work done on it?