Not alotta KTM motors out there though. I'd do either a KX65 motor, or an XR100 motor. The XR motor would be the easiest to ride (for a kid, or an adult), and it would let riders who want to race the series, but don't want to plunk down $3-5K to build a bike be competitive on an $800 XR100 from the classifieds. The thing is stone reliable too. I believe Service Honda can get a brand new XR/CRF100 motor for around $1000 from Honda. It would let $1500 NSR50's be competitive, along with NSF100's. Once you slap in a KX65 motor-- you have to sink alotta cash into an XR100 lump in order for it to compete... then sink alotta cash into the KX65 motor to compete with the KTM.
IMHO-- the XR/CRF100 motor is plentiful, easy to ride, inexpensive, and very buildable, and is the SV650 motor of minis. The KX65 motor would be good on big tracks, but it'll suck on smaller tracks, which is all alot of us have to race on. I could rip on my Metrakit racer on big tracks-- but the abrupt power delivery made it difficult to plow around tight tracks.
Your goal is to make the racing affordable, fun, and competitive. Put the XR lump in it! --L*64