Thanks for the Lotus Evora Conversion link! It seems like he did most of the work required to replace the pack in a first gen Roadster and remove the PEM. I'll try to find out and I'll post what I learn.
I like where your head is at - I too like the idea of taking the propulsion system of a crashed Tesla and retrofitting it into another cool car. The issue really is the battery pack enclosure design and motor location(s) of the electric car makes the design of the chassis completely different that the chassis design of an I.C.E. car.
Unless you could find a way to use the existing flat battery pack of the Tesla, a new battery pack enclosure would require a complicated arrangement of battery cells, wiring, and electronics, and would need to be custom designed and built.
That's just the first two basic hardware problems that would need to be solved. The software is a whole other set of problems. I believe it would require a team of software, hardware, electrical, and mechanical engineers to make such a thing happen.