For Forza Motorsport 4 on the Xbox 360, a GameFAQs message board topic titled "How do you do 2 player online multiplayer on Forza 4".
Post here which car you want to be converted and I will try to do it. So far I have done some FWD/AWD to RWD conversions for my drift cars EDIT: Here is just for fun an RWD and AWD conversion for the LEXUS CT200h which is Stock FWD.