I don't think Envemo built any 6 volt cars, ever. I would expect the transmission to use a 12 v starter as is and would also expect the 200mm clutch flywheel and pressure plate to fit just fine. Not knowing the engine size or how someone plans to drive, it is difficult to recommend a pressure plate or a clutch, for sure.
With that said, I have been living with a Kennedy Stage II with a solid disk clutch for 30,000 miles. The next time the engine comes out I am going back to a spring clutch and no more than a Stage I pressure plate. Just running around on the street, even with a big engine, I don't really need the inconvenience of the higher performace stuff.....a stage II or III in heavy traffic with a solid disk clutch plate ceases to be enjoyable in about 30 seconds....and after an hour you want to hurt somebody, seriously,,,,