The only difference is the RAVE power valve. But to add that you need a Senior cylinder block. New, the items run about $900 in parts. Now you can find these used. RAVE assemblies run between $75 and $150. Cylinder blocks anywhere from $150 to $400. I’ve recently heard that Rotax has relaxed the cylinder block casting serial number rules, so you should be able to use any year.
Now add to this labor and resealing costs. Thats going to run you another hundred or two. At this point you might consider a top-end rebuild just to match a new cylinder to the new block I.D. That will probably only cost you a hundred more.