thats super mad max, my razor dune buggy towed my car so im not shocked, , the motor on that has lots of potential when the kids a bit older, very cool build
id use an industrial brushed motor for these extreme requirements , ive used several.
http://www.youtube.com/watch?v=qZIPd5QY5a4
they will withstand extreme amperage and double to triple voltage , and are often internally fan cooled,but ur bike will be ugly and heavy