What is a 50cc sport 2 stroke? Used gear is super hard to come by. I actually had to buy Brenden a new suit this season since I couldn't find anything used. At 13 though it may be a little easier for you than me since Brenden is only 8. Just some FYI, depending on your sons size. 50cc 2 strokes are generally good to about 100lbs at most without sending the suspension out to get it redone for a heavier rider. Otherwise you could blow shocks and maybe even forks. Is he new to riding? If not you may want to look into a 65 with SM wheels. If he is smaller and you want a 50cc SM bike I have one for sale.
Brenden races with MinimotoUSA. If you want some info about racing or just trackdays and bikes let me know. I'd be happy to help.