There's really no need to rev any higher than 5500rpm with those anyway unless you just want more top speed. Even that isnt a good idea because any higher than 5500rpm is risking it, especially with the ohv engines since I dont know of anybody who has tested those for racing. By 5500 it will have hit peak hp. Like everyone already said exhaust really opens them up. To make the most of freeing up the exhaust you can put a k&n style filter on it. That would take some fabricating on your part though, because nobody makes an adapter for those. You could try to start out with a gx390 adapter (affordablegokarts.com) and modify it as needed. I dont know how that would work out though. Beyond that there isnt really anything made for the OHV subaru's that I know of.