Is the fender hole parallel to the wheel, or is it the downward-pointed hole between the fork arms? If it's the former, you can buy a caliper brake kit and bolt it on.

Second option is to build/buy a wheel with a front drum brake hub, but this would end up costing as much or more than your bike.

Third option: Find/buy a new fork with the brake mounting hole, like this:
http://www.amazon.com/Sunlite-Cruise...5918612&sr=1-3
Note the brake mount hole. You may need to cut the headset tube to length, and without seeing the bike I don't know if this particular fork is the correct size.