What would be the problem if your rm 80 2001 is reving high but not really hitting powerband?

From the factory the motor is jetted rich to be on the safe side. Assuming that there is still good compression... have the rings been replaced? The easiest solution would be to adjust the carburetor leaner. I would adjust the needle jet (the slider) to the 4th or 5th ring on the end of the needle jet. If this isn't enough, depending on your elevation you many need to put smaller main and pilot jets in the carb. I would go one size at a time letting the engine run at normal temperature before trying it again.

Jeff