The source for Rotax engine parts is Ron Wood Racing in Costa Mesa, California. He is the North American importer for Rotax engines. He specializes in dirt track racing, but supplies all engine parts and accessories. He should be able to help with anything you need. The contact information is as follows:
You may also be able to get some stuff from your local Bombardier/SkiDoo/SeaDoo dealer as Rotax engines are in their jet skis, jet boats, and snowmobiles. Granted most are not single cylinder but ya' never know until ya' ask.