I don't think you really need a disc brake... all you need is about any brake that came on any small dirt bike...
Why ?.. because most of them had 10HP or more... and they weigh more than a ped... both of which means they needed more front brake.
The thing will be getting the steering head to swap... and a front rim that you can find tires you like for it.. (don't know if you want a knobby on the front).
The other thing is ... finding a front end in a salvage yard that doesn't have fork tubes that aren't too rusty for your sense of style.
Do a search for 'motorcycle salvage yards' ... find one in your state (or nearby state?).. and take a visit and see what they have.
(first measure your ped for the largest steering head 'bolt' that will fit thru your peds steering head)
Then you might still have to do some tricky swapping of bearings.. and make spacers... and.. will the tubes come back and hit your gas tank at 'full lock' ?
I have been considering a similar deal... only I have a front drum off a 175cc trail bike that I might try to lace to a wheel of the right size... I will have 4 times the braking power if I do.