Hey, you've gone this far. You could go with one of the long stroke cranks and/or big bore barrel. I think you can go out to 850cc and near on 80hp using both. yes, I know it defeats the whole purpose of the bike; moderate power and good economy. I'll see if i can find the link.
This engine is used in the bombardier DS650 quad bike. So there are a number of hot up options.