If you have the tools to weld steel with, buy some v-brake bosses or a v-brake adapter plate, and weld them to your fork in the appropriate place(s). V-brakes are cheap and powerful. I prefer them over mechanical disc brakes.
The cheaper way out is to get some LONG cantilever brake levers (different pull than v-brakes, take note!) and use them with old style beach bike calipers. Weld a long piece of steel flat stock to the forks where you want the center bolt to mount, then hacksaw it off beside the forks and grind down for looks. Drill for the mounting hole, and you should be ready to install the brakes and ride.