I'm pretty sure that's a Honda CBR150R engine, it puts about 21hp at the wheel. Engines of that type are impossible to find though. Other option is to get a CBR125 engine and get the bore-up kit and DOHC conversion, both engine share the same bottom end. That's why I find this conversion is so appealing, the 6-speed HONDA tranny.
I took one look at the madass frame and noticed there was plenty of room for a vertical-type engine. There would need to be some modification done to the motor mounts for it to work. The bore-up kits, 180cc tops I think, aren't mind blowing like with horizontal engines but there's no denying that DOHC 4valve engines are amazing, especially Hondas. I was thinking of getting a Honda Innova 125cc engine and boring and stroking it up to 250cc or better, but I still run into the motor mount mod problem even though its a horizontal type engine.
Biggest you can go right now without fooling with the mounts is modding a yx160 - stroked out to 66-67mm with a 67mm bore which gives about 239cc. With the right tuning you can get +26HP. I also just saw that takegawa released a 4valve head that would work with 67mm bore. This summer is going to be very interesting for horizontal engine modification.
Personally, I would go with a Daytona 150cc bored and stroked and buy the DOHC 4valve conversion for a total of 4k and not have to mess with the motor mounts. Doing this would net about 23-24HP, but getting over 22hp out of ANY horizontal engine at the moment with require a fair amount of custom machining.
EDIT: Ark informed me that he used a Suzuki Raider 150R engine, which is very similar to the CBR150R but is air-cooled unlike the honda which is water-cooled. Honda is the way to go IMO because finding a CBR125 (JC34) engine and upgrading it to a 150R (www.akunar.com
) seems to be the easier route and its a Honda. Then you get the added benefit of a water cooled engine. Make sure you get the non-fuel injected version otherwise you'll need to install an O2 sensor and tool with the programming, which might be more than some bargain for. Other options are CRF motors but I'm partial to 6-speeds.