I have a friend who races a Westfield Seven. Apparently the way to go with those is a large motorbike engine because of the power to weight ratio.
He's currently using a 1000cc Yamaha giving him about 460bhp/tonne. I've ridden in it a couple of times and it's really scary.
http://www.fluke-motorsport.co.uk/pr...ml#GRAEMES_CAR
Phil.