Front: Here are 3 options:
Drill out the back of the fork crown (8 mm or 5/16 drill bit). This is actually quite easy to do with a handheld electric drill, since you're only enlarging an existing hole.
That's it if you can get two front calipers. Sometimes, you may have to deal with a pair of brakes, with one long and one short bolt. If you used the long one in back, you can use the short one in front two different ways:
Drill out the back of the fork crown and use an extra-long recessed nut. These nuts are commonly available for use in carbon fiber forks.
Use the short recessed nut, but don't put it through the back of the fork. Instead, push it up into the inside of the steerer from the bottom. You can reach a 5 mm Allen wrench in through the hole in the back of the fork, and poke the short caliper bolt in from the front.
You may need to shorten the recessed nut slightly to get it to fit inside your steerer.