Wow some of you guys have really offered up some quality info to this dude!!!
80HP out of a stock cylinder bike is not hard to get butthe question is why do you think you need 80? A good quality 4mill build will be in that 70 to 75HP range and is a hell of a fun bike to ride with load of torque. I rode a 4 mil stock cylinder for years and loved it. I now have a 421 cub that is 94HP and is still a bike I ride on the trails but it is much more of a handful and it is work riding it on the trails. I would call up a builder talk to them about the riding you want to do and see what they recommend you build.