big bore kit

My brother has a 2004 honda rebel 250. It is re-jetted, re-geared (higher gearing), and has aftermarket sidepipe exhaust. It is a sweet little bike. I was wondering if anybody knows anything about where I can learn more about a big bore kit for it, and maybe where one can be purchased. The goal is to get the bike to have a higher top speed. It will cruise easily now at about 65 mph, but we are aiming for maybe 70-75 mph. If there are any other modifications that any of you know of that will increase the speed of this bike, we are open to any. Please do not just say to get a bigger bike because that is not the intent of this post. The prospect of a little bike with a significant amount more of horsepower is what we are looking for. Any constructive comments would be greatly appreciated. Thanks a lot.

Are you maxed out on the sprocket gearing? The only thing aside from that bore kit I can think of for top speed is sprockets. If you don't need to get there fast that is...

for a big bore kit you could get it all done custom, where you get a machine shop to bore out the cylender, then machine new pistons, rings, etc. make sure that you keep the compression the same, or slightly different, because when you bore out the cylender the combustion chamber remains the same so it raises the compression unless you machine lower compression pistons or inlarge the combustion chamber.

you could also look into porting and polishing the heads, so you enlarge them a bit and remove anything that might be restricting airflow, then you polish them to allow air to flow more freely. after the easy little stuff getting more power, however small, gets more expensive. you can advance timing a bit, get the carbs re jetted, maybe a new camshaft.

what octane does the bike run on? if it runs on low octane you can run higher octane and more timing advance or more compression. also you could get a nitrous kit but then you'll have to refill the bottle and it won't help cruising speed, just acceleration and top speed for a short time.