Hey guys I just stumbled onto this topic and had to register.
I had a go at motorizing this set here using two XL-motors for the drive and one M-motor:
http://www.knicknacknock.com/index.php?title=8070#Motorised_Remote_Control_Lego_8070_Supercar_.28Modification_3.29
Its seems to have all fit in the existing chasiss, steps here: