I have never come across a Rotax engine in a nipper and it is a bad idea. The one weakness of the Rotax is it is non aerobatic, and the nipper is quite good at aeros. There are some Jab powered nippers, which are direct replacements to the VW Ardem and Stamo conversions. The Jab is aerobatic and has 85hp against the nippers 45 hp.
I had a VW 1834cc (about 60 hp) version which was good fun until the crank broke.
Rod1