I'll warn : part of the bad design of the standard rally / GL / sprint style fork is the fact it dives like a son of a bitch. If you improve your stopping power, but don't improve the dive, i would think it would feel pretty wonky ( maybe even unsafe ).
If you're trying to keep it stock - looking, but improve your braking, then make sure your brakes are all set right, the pads are good, and perhaps install an uprated spring in place of the stock one. The stock brakes work reasonably well if they are tuned correctly.
Everyone thinks you have to scrap whole systems...rip and replace...to make things better. The original stuff worked pretty well if you go back and set it all correctly.
If you're building a 25hp speed demon, then just bite the bullet and buy the PK fork and disc setup....or better yet, swap it with a lambretta fork and run a double disc.
Until then, leave well enough alone.
my 2 cents.