If you want to make a toy into something that can be run with hobby grade gear, be prepares to do some SERIOUS mods. Dave already addressed some of the key points. And not only that, toy RCs will break WAY more easily than hobbygrade RCs. You hit a curb and all the work and money you spent to use hobby grade gear goes up in an instant.

Honestly, it does sound like a fun project if you know wfat you are doing and you are a good fabricator. But if you only do a hak job to drop in hobby grade gear, I agree with what Dave said. You should just buy a hobby grade RC instead.