go turbo.
can pretty much bolt-on upto about 200bhp.
traitorrrrrr
there's loads of different chargers you could run, g-ladder, lysholm twin screw, eaton roots type, rotrex cetrifugal type (antonov make a two speed gearbox for the rotrex chargers)
there's quite a few options avalible turbo-wise aswell eg k03, k03s, k04, garret gt25 roller bearing turbo and thats just a few (you could bolt on whatever providing you have a corresponding manifold