I'd worry about protection and handling first. Skid plate, radiator guards, at least new fork springs to balance out the handling, revalve if you can afford it, bark busters, etc.
The harescrambles I've been to and raced in I never felt that I needed more power. That last one I was in I think there were 2 straights where I could get into 4th gear, but most the course was 1st gear stuff, extremly tight, just plowing small trees and brush for 3 hours (everyone I went there with quit after 1 lap, or less). Protecting yourself and your bike from trees/logs/rocks/roots was #1. Power is nice, but not really neccisary at most events, unless you are a pro rider and can handle it. I think a well suspended XR80 would do me fine in the gnarly stuff.