Over 10 years ago, there was an idea called "eFilm" that was a small digital sensor that fit into some standard film cameras. It was too expensive, the resolution was terrible, and the FOV crop was extreme. Also, the whole thing may have been a scam.
These days, you could do the same thing much more effectively with a FF sensor. It would cost a few million USD to develop, but you could probably sell it for around $2000. Obviously you could make that back very quickly, as there are at least a few thousand people who would buy a device that would turn any old 35mm SLR or rangefinder into a FF digital body. The problem is explaining this to an investor in easy to understand terms.
Honestly, you may be able to do this via Kickstarter. A game project recently cleared $3.3 million USD, so you could raise the right amount of money. The biggest hurdle for the Kickstarter project would be getting accurate quotes from the professionals who would actually make it happen.