I had the same problem converting a pair of these brakes, as the standard replacement spindles available are not compatible. I came up with this solution (some may say botch), but it works well.
Fit the front brake to the rear - you'll probably need to turn the brake blocks around if they are directional.
Get a long recessed nut and cut the allen key fitting off. Attach this inside the fork tube to the brake spindle which should be about half way along the nut's thread. Screw a length of spindle in to the other end of the recessed nut and this extends the spindle. Use strong threadlocker on the recessed nut. Mine has been really solid and no problems so far.
Too many bikes and still too few