This is about as far as one can take suspension, chassis & braking without making huge changes that go beyond the stated scope of the project. Between the front ends mentioned in #152, it's a choice between better braking or suspension. I'd have made the same choice; a well-tuned K1 front end is surprisingly competent, the low-buck inverted fork is too soft. Unless you really like the aesthetics, or just want an inexpensive disc brake, it's more of a tradeoff than an outright gain.
This chassis setup should be pretty good to about 60mph, over decent pavement. The front drum brake is competent, IMHO, to about 55mph. Above those speeds, I'd want more fork travel...or very smooth pavement. And with a little rider skill, engine braking will yank the bike down below 55mph pretty fast. Then you can just grab a handful of brake, as needed, drama-free.
There are better front ends out there and kirrbby's twin-piston-caliper/large-diameter disc brake can deliver at least double the braking power of the stock drum. It's also effective to at least 80mph. I've had to execute a few real-life panic stops, some while riding two-up. That hydraulic front disc is in a another league. It doesn't mean that it's the only setup worth having. I ran stock drums front & rear, until 2004...about 4,000 road miles...and lived to post this.:3: Not everyone wants, or needs, the same things from a bike. Thus, there is more than one "right" answer.
FWIW, speeds above ~65mph are only good for short bursts, on these little bikes. And one can get away with a lot for the occasional short banzai blast. 80mph sounds like more fun than it really is, with a machine of this size & mass.