Post a photo of your whole bike please, especially the wheels. It will help a lot with suggestions.
Coaster brakes are very dependable, especially on older bikes like yours. I do understand wanting extra braking, however
Don't be afraid of drilling the fork. No matter how complicated some try to make it, the job comes down to measuring and drilling ONE hole. No PhD involved. The hole you see is the machine target for drilling it for a caliper if needed.
Most forks other than flat blade solid steel Ashtabula style (old Schwinn) forks are the same fork whether they were drilled or not. That bracket you show is for one of those forks and probably wouldn't fit properly on the fork you have. The other manufacturers rarely made two forks to do one job, they made one fork to do two jobs.
Same with fender bolts. One bolt for both jobs. Most of the fender mount bolts are vertical drilled as well. Unless the fender bolt has been changed for something longer due to loss of original bolt, the bolt will clear the caliper bolt. If it is long take it to a hardware store and buy one that matches with an Allen type (internal hex) or standard head, not a screwdriver type head. You will thank me later if the bolt gets rusty.
The measurement is already there. DO NOT try to hand drill both holes in one action as it is hard to hold the drill perfectly straight. If you drill the holes as I suggest you can run the drill bit through both holes together afterwards and align the holes within tolerances. Or use a drill press, your fork bearings might like a cleaning and some fresh grease anyway.
Modern BMX bike steel calipers properly installed, with good pads (more on that below) will work as well for your purpose as higher cost calipers some may suggest from race bikes or new from the local bike shop. They will also clear middleweight fenders fine. If you are into it (I am) you will find what you need on one of the millions of new bikes Americans discard every year because the tires went flat or the paint color is out of style. Just don’t use the pads, they are most likely made from rust and cow dung.
Like another member said, the caliper ends can be gently bent to align with the wheels, and on steel calipers this could be done to align them to drop center rims as well, although I never had to do so.
A lot of bikes came from the factory with drop center rims and caliper brakes. I don't like it either but they work after the pads wear in or you sand them on a belt sander to cut an angle horizontally on them.
Buy good pads not the cheap things at Wal-Mart. Look for the RED ones, they are a better stopping coumpound in the old pads and are better wet than the black ones. Buy some nice ones from eBay, or I may have a spare set or two.
Another solution (not my favorite) is the bolt on caliper brake adapters. They bolt onto the fork legs at any point needed and are still available here and on eBay reasonably. Schwinn and Resilion both made them, the resilion version may cost less but it may take a week or two to get them from England. They use a cable similar to a GYRO cable on a bmx trick bike. I do not like them because they scratch up paint on the fork and if tightened too much or the set bolt tightend they can dent the fork a little.
Drum brakes are nice, I use them on some of my musclebike builds on frames that arent made for caliper mounting (ie: Schwinn) but the Schwinn Fore Brake, while an excellent choice is way too expensive to use on an old middleweight fun bike. It will also make the bike, wheel or even the hub (by cutting the spokes and brake cable) prone to theft. I would suggest a Sturmey Archer, Akai, GRI.ME.CA, or even the knockoff wheels with drums installed. There are cheaper ways than eBay if cost is a factor (it is for me).
Feel free to contact me or I am sure some other knowledgeable members who will be able to walk you through this, or check with the bike co-op. Someone there I bet spokes wheels and may build you a wheel for parts cost only (I would but too far away) or even a modest fee that would be worth it and still way less than a Fore Brake alone. If you like that enough buy a rear to match and put a single speed BMX sprocket on it, or if you use Sturmey Archer get an AB hub and have a 3 speed with minimal setup.
Not a sales pitch, but I have a spare AB hub here to do a 3 speed rear and can get front brake hubs from England if you decide you like the idea. Nice shiny chrome, and everyone knows that while chrome may not (always) get you home, it may get you somewhere. And it is fun to polish!
I also have a GRI.ME.CA 36 hole rear here, but they aren't as shiny. What I did see of your bike says SHINY!
Am I at 5 pages yet? Just checking.
Rob
My name is Rob, and I approve this instructional.