Good luck with the new engine idea Axel. How about doing something quick and easy for now and putting exotic ideas on hold for a while? For example - complete Boxster engine and box - no adapters to worry about. Audi V6 with dressup kit. Honda Legend V6 (auto only). Convert to Transverse and use an Alfa Busso V6.
https://engineswapdepot.com/?p=25774
I'm sure with your skills you take that plastic engine cover off and put some fake trumpets on!