Its been done in Germany for the 150,
Rotax engine for Cessna 150
The Rotax engine conversion for the Cessna 150 has been certified by the German aviation authorities LBA since October of last year. This could be accomplished mainly to the long-time effort of Dr. Guido Sperl, head of an engineering office which is specialized in structural, dynamics and fluid mechanics development work. The 56000-Marks modification is exclusively done by Fläming Air located at the airfield Oehna/Zellendorf (phone: +49-33742/60330). A modification kit for the 100-hp Rotax 912S3 is in development also, the certification is expected for the end of this year.
Above from aerokourier magazine.