Keith – the significant changes are clutch, airbox, and a tightening of cylinder head castings tolerences (some pre-2009 cylinder heads are now illegal for official RMC events). These are the basic things that will need to be changed/checked for RMC entry. The clutch and airbox are no-brainers; the older clutch had reliabilty issues and the new airbox makes removal (for jetting) much easier. The cylinder is something to check your casting number against the rules. Do a search on Rotax cylinder casting and you’ll find the casting numbers that are legal.
Changes Rotax makes to the engines over time are designed to not improve performance. Instead, the focus is on better performance consistancy between engines, improved reliability, and easier maintinance.
Other changes made are (optional*) hotter ignition module for more consistant high RPM performance, needle bearing and o-ring for clutch to prevent bearing grease from getting onto the clutch drum, improved battery options, and most recently, the reduction of carb venturi options from two to one (8.5).
* EDIT: Since this thread has been getting some traction recently, I wanted to clarify something I wrote to avoid confusion. When I say the hotter ignition module is “optional”, that means the newer 2009+ coil with the green tab is NOT a required upgrade. All newer engines have this coil, but it is not required to upgrade an older engine for RMC legality.