A lot, can be done, but its expensive, get in touch with this guy:

http://www.davebrooks-engines.co.uk/

Think I've heard of 10.5k, but basically all steel, possibly dry sump, high compression, big valve head (something like 44.5or even 45.5mm inlet valves, say 38 on the exhaust), raised port chambers for better flow, a proper lumpy cam, and twin 48 cards (50s tend to slow down gas flow), oh and a good tubular manifold.