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Yes, it is very possible. I have seen proof of a CG250 engine making 18whp on a bike, which you could roughly translate to 20 crank hp. What it requires is head work. the ports on these heads suck, and I have a whole thread with pictures showing just how much they suck. The engine in question I am referencing also has a decked head though, which bumped the compression up almost a full point. While I do plan on porting the head, I do not plan on decking the head to that extreme. I will aim for maybe an extra half a point at most. More compression = more heat and stress = reduced life.
This is in fact my goal over winter with my Hawk. I already have an extra head, I just need to get around to doing the port work to it and then taking the time to install it.
The cylinder heads themselves are actually not that expensive. I got a complete assembly for $50, and I regularly find them around 50-60 bucks.
The expensive part, if you can't do the port work yourself, would be having a shop do the work. Thankfully, since it's a single cylinder 2 valve head, they cost won't be too ridiculous. You could probably call around and get quotes. Likely cost would be around $150-250 depending on what you want done.
With a tuned carb, free flowing exhaust and intake, somewhere around 17-18 crank hp is realistic for a CG250 engine. If 20hp is your goal, the last 2-3hp has to be made up on the cylinder head.
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