The power valve (or exhaust valve) opens up proportionally with the engine's RPM.
I haven't seen the mechanism properly but seen photos of it. I'm pretty sure you'll be able to add some washers or spacers to limit the travel of the mechanism, thus preventing the exhaust valve opening fully and in turn softening the power at higher RPM.
You could also experiment with disconnecting the linkage and locking the exhaust valve in it's closed position. In effect this lowers the exhaust port giving more power at lower RPM and less at higher RPM.
These are a bit expensive, but might be exactly what you're looking for...
http://www.enduropuro.it/shop/en/gas...lve-cover.html
Good luck, Dave.