This is the best new 450 6-speed engine I've found new. Almost $1000. Wouldn't be a straight bolt in, I don't think. But it would be 3x the horse power. Which might be a bit scary on the Hawk. You would probably need to get new brakes, disks and sprockets at the least to be able to handle it.
http://cqskill.en.alibaba.com/produc...dirt_bike.html