I haven't done it, but I did spend some time thinking about it. Generally, the engine wouldn't be that much trouble if you can get it from another kit. You can probably build the 'chassis' from the kit piece with some basic skills. Suspension is pretty easy, but the real tough part is building all of the surrounding pieces that frame everything (like the engine compartment, as naplak mentioned above.)
Depending on the car, you might want to check if there is a resin 'conversion' kit for your model. This might be easier than trying to do it all from scratch.
M.