I got this idea by looking around the internet at different bikes where I came across a Vespa GL with a disk brake on the East Coast owned by John Irvine. He built his bike with a cable operated master cylinder in the glove box, but recommended a full hydraulic set up if I didn't mind changing the headset. The forks he recommended are from a Vespa PK125 which was never sold in the States so I contacted Beedspeed (a shop in England) and had a set sent over. The disk brake kit is an aftermarket part that can be added to either PX, T5, or PK forks but as far as I know it is only made for the 20mm front axle. Older P series bikes have a 16mm front axle. If you are unsure which axle diameter you have click here for a brief description on how to identify a 16mm axle and a 20mm axle without removing the front hub.
The PK125 forks have a 20mm axle and although the VBA mudguard (fender) has to be re-worked (see below) to fit the new forks, the forks themselves drop in to the VBA frame without too much hassle. The steering stops had to be changed but the fork stem length was correct.