Max continuous performance

Just for reference

This Cessna C150 was equipped with a Rotax 914 and flew to 24,000 feet hundreds of times during testing of the engine in the early 90s. As you increase in altitude the performance of the engine will diminish and manifold temperatures will increase. Keeping your parameters within limitations will dictate the max altitude you will be able to operate at. 

I would love to know in which aircraft you are flying to that altitude with a 914.  Pressurized cabin?

Critical altitude is usually defined as the point above which the turbocharger can no longer maintain maximum rated manifold pressure.  I would suspect you will not get 40" of manifold if you try a max power climb (for 5 minutes or less) above 16,000 feet.

As far as impacting the engine, I can only speculate.  Typically engines have difficulty shedding heat as they fly higher.  I'd expect Rotax is not much different.  That extra heat impacts thing like rubber parts, and engine mounts.  CHT may hit limits, and the induction air may also hit limits since it is much hotter.   Also I'm not sure how Rotax ignition modules handle the altitude, but typical magnetos can arc more at higher altitudes due to the lower air densisty.   Another limit you may hit is the max RPM (and well below 16,000 feet).  Presumably you have a variable pitch / constant speed prop?

Another issue you might face is excessive engine cooling if you "chop the throttle" and glide down from 24,000 feet.  Power on descents will solve this problem and let you make up that speed for the slow climb up to the flight levels.   --And yes, you start planning descents 45 minutes out. (100 miles out.)