Hi Robert
Can’t put my hands on the Haynes manual at the moment, here’s a pic from the BMW manual.
https://drive.google.com/file/d/1lv9gSIzvaFLge9c7oYwzTdIKLjt7znd4/view
The cover 4 measures about 64mm dia.
Also the Harris G80 used the Rotax 500 engine.