It isn’t a non-starter at all - a lot of experimentals are using auto conversions very successfully. The devil is in the details, of course, do it wrong and you have an unreliable engine. But if individuals or small groups can do it successfully, then a fully engineered solution should be trivial. But it would require the expense and complication of certification, which has killed many such attempts already, either directly, or by forcing the engine prices so high that ‘I might as well buy the tried-and-true’ engine. Similar reasons why you can still buy brand-new PA-28s and C-172s!