I'm trying to upgrade the brakes on my commuter. Got two Proponent BMX brakes from the 90's. Got two rears and I'll try exchanging one for a front but I bet they don't have one. Problem is, the rear bolt is too short to go through the hole in the unicrown fork.

I am trying to find/modify a bolt that will fit. The photo shows the rear bolt. I can find shims & washers from another old brake. But, I need a bolt with a low-profile head and an anti-rotation shoulder like the one in the photo. Or, a low-profile hex-head bolt & then I may have to get a low-profile wrench to fit. Does anyone have any ideas for me? TIA.

Longer story:

Don't want to give up on this brake as it is supposedly a very strong stopper. I like the bike, with its low BB and fat tires. It's surprisingly fast & its cockpit is perfect commuter fit for me & am very comfortable on it. That's worth a lot even if modern bikes have better brakes & stuff out of the box. I would continue to upgrade it until it's hardly recognizable.

The bike is a 1987 Schwinn Cruiser Supreme, hi-ten Giant-built I think, 26" mtb wheel size). The LBS several months ago sold me two Proponent brakes. I didn't try mounting them 'till yesterday. I didn't notice until then that the bolts & washers were both rears. Google shows these as long out of production. I bet the LBS will be unable to help but I'll go there (no reply to email.). More modern designs have made big-reach, wide-jaw brakes like this obsolete; are any good ones still available new and at a reasonable price?

Other options - cost effective or not? How much?
--Braze cantilever studs onto the current fork.
--New fork. Don't want to upset the geometry, love the handling as-is.

TIA