First off, you're going to catch a lot of crap for reducing the performance of the already underpowered braking of the 750.
We've seen people try to put the drum brake from the cb450 on. Dunno how easy it is.
Nothing will be a simple bolt on. If you really want a drum brake up front, try something from a BMW /5, a moto-guzzi, or other LARGE, powerful drum brake. Most CB750's are close to 500lbs wet. You'll need a good brake.