1. The Cheap But Difficult Option - You fabricate all the needed parts yourself, including modifications to the Beetle. This is time consuming and requires intimate knowledge of Beetles, Corvairs, and mechanics in general.
2. The Expensive And Not As Difficult Option - Get a conversion kit. Search for one on Google or look in a VW magazine. You will still need a little knowledge of Beetles and Corvairs, and a lot of general mechanical know how.
3. The Really Expensive And Super Easy Option - Pay a professional to do it. There Are several shops able to do such conversions, but be warned that they are not even close to cheap, reasonable, or sane.