That bike would require quite a bit of effort to install disc brakes. You'd have to fab up caliper brackets on the forks and get your wheel relaced with spokes and a hub set up for a disc.
This kit would probably be the most reliable 4 stroke motor with a shift kit.......doesn't require the use of a chain on the left side and also it allows the use of your rear gears. Many members have also fabbed up their own version of this kit.
Good luck whichever direction you should take!