Get a Webster Whirlwind kit? Ron Webster thinks he can make up a kit of machineable parts for not much more than a grand. 5 cylinders and about 75bhp from cheap parts. Google for G-BMOO or youtube for Webster Whirlwind. It uses VW heads etc (see above, message number 2!)
Ron Webster and G-BMOO | Flickr - Photo Sharing!
Clutton FRED 2, G-BMOO, PRIVATE
YouTube - shedworld99's Channel (Me in the yellow jacket)