I have a KX21 motor on my kart, we race these over here in a sealed engine class. Very durable motors with about 10hp and rev to 6000 rpm nicely, more if your stupid. Like another poster said aftermarket is close to non existant compared to clone engines. I'm switching to clone motor just so I can chase some power, although the subaru is a much nicer motor very few parts suited to a high rpm build are available.