Hi Ren, Rear brake kit part number AC-211NE-GB403 [retail price $36.60], and Earl is correct you need a heavier brake cable the part number is #2911 [retail $9.95]. The only comments I have to add to Earl's outstanding detailed instructions are, the hub will tighten it's self when the brakes are applied, because the rotation of the wheel is in opposition to the hub threads, and secondly moving the axle is sometimes difficult because of the small thin jam nut against the cones [I had to make a thin wrench to fit]. These parts can be purchased through any Authorized Whizzer dealer, but I give an average discount to members of this site of 15% on almost all items, except for modified speed parts [already priced at rock bottom].
If anyone is interested in a Whizzer front disc brake system, I might have access to a few complete front ends [forks, wheel, 3" tire, fenders & braces, disc brake rotor, disc brake calipers, and heavy duty brake cable], but must warn everyone this is the best brake I have ever seen on any Whizzer in my life, bar none! Just PM me if interested and I will pass along details [if still available].
Have fun,
Quenton