Motorcycle Questions? Ask a Mechanic.
The first thing that I would look into is a compression test.
Sitting that long will sometimes cause the rings to stick in the grooves on the piston. If that happened you'll have low compression which means lower horsepower.
You could pour a little marvel mystery oil or atf fluid in the spark plug holes and let it set for a day or two to see if they come loose and the compression comes up.
That's if it's low.There's not really anything other than the carbs that could cause a lack of power and since you've had them cleaned I'd say the compression being low would be about the only thing left as long as it's running on both cylinders with no misses.