Apologies for being long-winded, but here’s my .02
Since neither fork on the Monark-style is bolted to the axle, there is no constant distance from the rim for a canti post to work.
However, with a Schwinn-style springer, like you have pictured, the fork is bolted to the axle and a constant distance exists.
So, you have two front brake options with this type of springer.
1. Use the hole typically present at the bottom of the steer tube and bolt on a side or center pull caliper type brake.
2. Find a springer with canti bosses already welded on. Typically, canti brakes provide better braking force than calipers (that’s why they were invented,right?).
So, now you have some things to consider.
Option #1: Easy to find the springer. However, they are typically a 1" threaded steer tube, but can be found in 1 1/8” (see link below) I can't tell from the picture of the bike, but if it's running a 1 1/8" fork now (either threaded or threadless) you going to have to install (perhaps purchase) a 1" threaded headset and mix it with your 1 1/8" cups/bearings. Ignore that last part if you buy a 1 1/8” unit.
Also, you will need to source the caliper brake (as in, perhaps purchase).
Possible expenses to install option #1:
-the fork (your headtube looks like a pretty standard length, so finding one to fit shouldn't be a problem (cost varies on make/model)
-1” headset (cost varies on make/model)
-caliper brake (cost varies on make/model)
-new 1” threaded stem (cost varies on make/model)
Option #2: Cost of the fork is a little higher, but you can reuse your existing brakes and headset and possibly stem. Below are links for both a 1” and a 1 1/8” fork with canti bosses (you may need a 1 1/8” down to 1” stem adaptor, if you go with a 1” fork).
If it were me, I’d go with option #2 with a steer tube that matches your existing stem and headset.
Links:
http://cgi.ebay.com/SPRINGER-FORK-2...POSTS-NEW_W0QQitemZ380238605961QQcmdZViewItem
http://cgi.ebay.com/chrome-springer...Cycling_Parts_Accessories&hash=item414f9b084a
http://cgi.ebay.com/Springer-Fork-2...Cycling_Parts_Accessories&hash=item27b6935640
Good luck, I think that frame would make for an EXCELLENT boardtracker style bike. It has the look of a ‘tanker’ bike, while being modern.
Cheers,
Dr. T