XR50 questions

I just bought a 2000 xr 50 and a couple questions have come up that I hope I can get some answers to. Lil background, my son is 9 and rides a quad now. I want to get a couple bikes for him and I to play on and do some back yard chasing and start into the trails. I wanted something that was comparable so he and I could "race" and be competitive. Me on a 80cc and he on a 50 would be a pretty good match was my thought. The XR50 has so many mods you can make to it that it seemed a perfect answer. As he gets older I can keep hopping up the motor until he outgrows it. I also want the ability to convert it to motard function if the opportunity permits.

The sticker says max weight 88lbs, yet I see these are ridden by "big kids" all the time. Do suspension upgrades have to be done to handle the extra weight or is that the safety nazis being overly concerned?

I may have taken it in the shorts on this bike. It was advertised as having a Lincoln 125 motor freshly installed. While bigger than what I needed, I figured it was a nice thing to have and I could swap in a 80cc and keep this engine for later. When I picked it up another guy was there, not the seller and he said "that motor is stock and has never been replaced". How do I figure out what motor is actually in there without tearing it down? I have looked at some stock motors and this looks different for sure.