In Leymans (sp?) just bore it out,increase the stroke, drop the compression ratio and toss in some hopped up cams and forged pistons and load a fat kompressor on top, lysholm type is my personal favorite but they're kind of uncommon. Naturally, air and fuel need to be compensated for, contact a chiptuning place, powerchips, renntech, etc. and tell them what you're doing and they could mod your existing ECU or get an aftermarket deal. You probably want to know a good machine shop, too.

It should be pretty good up to 14 PSI, but to be safe, and with all the other stuff, 10 or 12. That should be plenty. Example the stroke only needs to be raised a smidge, couple thousandths, and the displacement comes way up along with HP. Just dont tell anyone about increasing the displacement, so as not to pay for it, I guess, i dont know how it is over there.