If you have a new spark plug and the carburetor clean and perfectly adjusted, I don't think you are going to find much more power. Either get used to it or buy something bigger. I went from 400 and 500cc Cats to eventually 2 700 Cats, 1 Diesel and a TRV. Even the old 2004 400 Cat was pretty fast, btw. No replacement for displacement.
I doubt you will find a BBK for that machine. Like was already stated it takes cc’s to get power for the most part with quads. You can also spend a **** load of money and still not be fast enough.
How long have you been riding ?
the cost of adding a big bore kit IF you coudl find one, wouldn;t be worth the costs for e small amount of HP gains there would be
money would be far better spent saving up for a larger motored machine, which will most likely also get you a better handling machine, with many more options for upgrades down the road
now another option might be,
not sure what exact model you have, to know for sure how easy or if can be done!
but there might be gears you can change to get more top end speed if you want that, won't be much more, and will come at a cost of lower end power,
but might be cheaper and easier to find deal for you to do!