Supertec head stud kits are the best option for all air-cooled 911 engines.
There are other options that work but none of them offer the benefits of the Supertec head stud including ease of assembly and life time warranty.
They are available from our host.
https://www.pelicanparts.com/More_Info/SPTCHSK1.htm?pn=SPTC-HSK-1&bc=c