check out bisimoto's build of the single cam. he's making 650-700 hp in his.
just search on some sites, and find what you need for high hp.
but its just gonna become a money pit. you'd have to get new rods, pistons, possibly sleeve the block, clearancing?, better valvetrain, ported/polished head/ bigger intake manifold, header/turbo manifold, big ol' intercooler, big turbo, lots of tuning. but then you'd have to upgrade the clutch/flywheel at the very least.
but good luck on that. put up a build thread if you decide to do this.