The motor has a serial number, doesn't it? Or are you going fabricate your own?

Probably may not even be road legal. There are also several safety factor that would come into play. I don't think any prudent insurance company would even want to give you cover if they know of the change.

Vehicle technology has grown and many advances have been made to enhance safety and fuel efficiency. Present day engines are carefully intertwined with other components to achieve these objectives and you would need to undertake a very expensive research (worth many cars) to achieve the same safety features with a different engine type.

If however you are convinced that your approach will lead to production of cheaper or better car than the ones in the market, maybe then you can approach the institutions of higher learning with a proposal e.g. recently launched business innovation centre at KU.
If you build the model at home using Jua Kali, then whatever you come up with will end up at the police station…and that’s where it belongs. Ask the Nyeri Man who built the Aeroplane