The right turbos and enough boost will get you 700+ out of almost any engine. Guys are making 1000+ out of reasonably built small blocks.
My little ol' small block makes 800+ and it does it on nothing more exotic than Optimax. Mine has the bonnet scoop from hell, but it could all be made to fit under a standard bonnet.
Best budget idea for big HP is Lexus 4.0 with a couple of turbos. Figure on between 500 to 600 out of a well tuned internally stock motor. That's more then enough for a Cobra. It'll make your Minis feel pretty tame
