Supermouse3: STC's to allow them to run mogas would be cheaper
There are STC's already. Look here;
Petersen Aviation | Auto Fuel STC
People are already doing successfully the things you say cannot be done or will be difficult to achieve.
I don't see Lycoming nor Continental avgas engines being knocked off their perch any time soon. A few have tried but none have succeeded. The fact of the matter is it's very hard to improve on the figures already being achieved with these "old dinosaurs". Certainly nowhere near enough gain to make it worth while.
The only option that has gained any traction has been the odd one or two diesel engines and their merits are very doubtful as a replacement option for an existing airframe.