My 10yr old son has been racing trhese types of quads for 3 years now. We have some prototypes and modified units based on this model which we race. we run 125cc 4-strokes that are bastage engines from different models being made to fit for our needs for each specific track type. Some are for top end and some are for hilly terrain. (we do MX and CX both.) I will get some photos and details of what we have and do for you guys.
The good is i love modifying these things and making them purr and my son loves tearing them up on the tracks(lol)
The bad is he costs me $$ we go through 3-4 race engines a year, and at an average of $900-$1200 in parts and machining that's not cheap! but racing is really not for the faint of heart. we compete against DRR/Apex and Cobra ATVs. truth be told. 90% of it is rider ability at his age and classes.