You need to read the Rotax manual before attempting. Also, you need the specific compression tester for Rotax. Not one for antique engines. The orifice size is different. A Rotax 9xx should loose little compression through out its life, if cared for correctly.
The orifice specified for the tester for a Rotax 912 is .040. That is the same diameter that is used for a Lycoming.
The aviation specific testers I looked at needed only the correct spark plug adapter.
We made our own by breaking apart a used spark plug and welding an air fitting onto it.