To me, there's a huge difference in a 904 and stock America or Speedy. I had the BB done years ago and have been able to tweak out dyno runs of almost 70 hp and 66 ft./lbs. torque. Also made a big difference riding 2 up.
My 904 is for sale in the classifieds section. Thought I'd never give it up, but getting older and can't have two bikes. I'll keep memories and pics but it will be gone next week.