You have the bike there. Measure the hole to rim distance and then see if you can find a caliper brake of comparable reach. If a brake exists that's reach, then it's only a matter of working out mounting it. The brake needs roughly 3/4" of usable surface at the front of the crown with a 6mm through hole in the middle. If the crown is tall enough, you might have to modify the hole, or maybe drill it through to the back, but these are only details to be worked through.