Serious modification is definately the way to go for increased fun - even if it does restrict where you get to use it.
I could supply you with a bodyshell (minus the lights) and a standard chassis with its steering components, but it would probably be more cost effective for you to buy a complete c5 and sell off what you dont need. More importantly though, I would be very careful about putting such a powerful motor in a standard c5 setup.
Take a look at the two project c5's built by c5alive & Dragon Technology named "Challenger" and the "C-5000";
This is "Challenger";
http://c5alive.co.uk/forum/index.php?topic=559.0..which started life as this;
http://c5alive.co.uk/forum/index.php?topic=266.0..and this is the c-5000;
http://c5alive.co.uk/forum/index.php?topic=1034.0Challenger is running a 1kw motor, and the c-5000 about 9kw. Note that neither has the standard chassis,mainly because the original Sinclair item simply isn't strong enough. The steering linkage is too feeble , and the brakes only just cope with the standard 250w motor. As coverman says, you may well get a few more snags with the other c5 "transmission" components when placing that much power through them.
I would urge you to consider building a much stronger rolling chassis and place the c5 bodyshell and other Sinclair components on top.