Unusual for it to come with a 15, Nuva Faor were selling the nitro bike kits in the US with a Sirio 12 which is a bit weak really. I brought one back from Vegas and tried it at the next UK race meeting in the dinner break and it was way slower than the Novarossi 12 that used to come with the kit. Pullstart problems and lack of power led me to ditch it in favour of the turbo head Novarossi 15 which is expensive, but ballistic by comparison!
But having said that, the Sirio 12 is probably a good 'starter' engine, then once that wears out, most people would have the experience by then to handle a motor with a bit more power.

John