YIts my understanding that you didn't have any issues with it running, only with replacing the bearing and installing the drive. If the gas had sat for a long time (year or so) then yes you had some old gas and probably needed changed out.
Now if the gas was just from last season then, the guy drained your tank and put it in his boat or car, then put new gas in it.
I don't have a warm fuzzy with a guy that want's you to use seafoam, IMO that's snake oil. There are a few here which will not agree, this is just my opinion.
I would just chalk it up to now its fixed (we hope) and maybe find another mechanic for if there is a next time.