since every one has given up on the roatry engine idea I have another one
have you seen the cars that are about the length of your hand the rc ones they have at radio shack
well put a cox black widow style engine in it it has a built in fuel tank the forward and reverse could control the reed valve and it would last for around 5 minutes
you would have to make a alluminum heat shrink so you dont melt the car and you wouldent be able to stop but it would be cool and fast as hell from what i can tell with a little bit of gearing