you're pretty much right about the water cooled CAG... they only made a small handful of them and they had around 6 to 8 hp, they were cheap but Fast.
Blata is a company that put quality in the pocketbikes, a Blata Origami costed nearly 3 grand but it had a 40cc liquid cooled engine that put out 14hp, then they made a larger one called teh Blata Elite which made 16 hp from a 50cc engine. then the Chinese copied teh engines and offered water cooled Cag bikes for about a year or so back in 03/04 timeframe. they really didn't make a big hit on the market and by this time those bored politician's wives were hot and heavy about banning pocketbikes from all public roads, and the liquid cooled variation wasn't something you'd run out and buy for your 10 year old to put around on since it could easily push an adult well above the 50mph mark.
The genuine Blata Origami and Elite series can easily push a 200 lb adult into the 60 mph range with 14 to 16 hp on tap and at 15000 rpm.
i've looked for Blata engines online to try and get one for a mtorized bicycle project by connecting to a CVT and gearing it to go in the 45 to 50 mph range, but these engines are either not available or not cheap when found. A lot of the left over Blata engines were sold as RC boat engines because of their tiny size and ultra high output.
Here's the specs on a Blata Elite 14WRS which is one of the strongest ones they make...
You can get them as a full race version like this one either naked or with full fairings, and they also make detuned versions for younger riders as well as one simply called the 2.5 which meant 2.5hp air cooled, but very well built thruout.