Drivetrain:
It shouldn't be a chain drive....... u need a differential....... CNG rickshaw differential is just the right size for you. The hurdle is to mate the differential with the motorcycle engine. Not easy to be very honest. CNG rickshaw ka engine lena paray ga. On the upside Its water cooled but downside is...... expensive.
Engine:
If you cannot afford a rickshaw, Get a 150cc air cooled Chinese engine from the scrapyard. They are pulled off ATVs and have a reverse gear too which is essential for a Car.
Frame:
If you build a frame yourself from ground up....... it wont be road legal....so cant drive on road...... SOLUTION: Get a old 2 stroke frame on which you can mount the engine and build a tube frame chassis around it.
Front end:
Get a complete agla hissa of a small car....like a mehran or something. Steering column...... front suspension, hubs, brakes etc. You will have to reduce the width of the front to make it match the rear end of the rickshaw.
Body:
Fiberglass..... Its light weight. You can make your own panels...... material you need is glasswool sheets and some resin........ you can search for the exact recipe....... for the molds..... you can dig a hole in the ground and shape it to make you mold..... ppl do it. It works.