The CB550 engine responds to the same hod rodder tricks that work with other 4 stroke engines.
You can up the bore with larger diameter pistons increasing displacment.
Domed pistons will up the CR. You could go to 10.5 to 1 insteads of the current 9 to 1. It should still only require pump gas, albeit higher octane. But, this will make more power, too.
http://www.megacyclecams.com/These people will make you a hotter cam for more breathing capability.
A freer flowing exhaust will also help breathing (more HP) at that end.
Almost anything you do above will require fiddling with the current carbs or adapting newer carbs with larger throats for added power increases. Unless you have a dyno or lot & lots of time to play with the multiple adjustment inside the carbs, the carbs will be the most difficult to dial in to any modifications made to the engine for horspower.
I notice you didn't mention how much money you wanted to spend or the horsepower gains you are looking for... Adapting a turbo would give you a hefty power increase!
I'm kinda with the why bother camp, though. These bikes, when properly tuned, are quite a nice package as they are. The handling characteristics are definately 70's. And, more engine power will magnify all the limitations of that era, promting even more mods to the rest of the bike. You'll impress people at the stoplight grand prix with engine mods alone. But, a modern 600 class machine will be more than your match even with a couple (or more) thousand dollars in mods, especially on the twisty roads.
That doesn't mean you shouldn't do it if you want a wrenching and learning adventure that can be rewarding in its own right. Just be careful what your benchmark for success is. Otherwise, you may curse the day you touched it with a wrench.
But, if you are wimping out at the prospect of spending more than $300. I suspect all you'll do is talk about it.
Speed costs money.
Cheers,
