A compression test would be a good start. I cannot comment with any authority on what you should expect from a Rotax, but when I test my stock Hondas, I expect to see about 120 PSI. Get a baseline and test again after running for a couple days. If the pressure is dropping below your baseline then the ring may be due for replacement and it is probably ready for a new top-end.
You may also want to post in the Rotax specific forum further down the page.