Bike has cantilever studs on the frame and fork. Since the reach is in the wrong direction, you cannot use any brakes that mount on the cantilever studs. Going from a bike designed for 650b with cantilever studs to any larger rim size can be accommodated with the appropriate brake (Paul Component's Moto-lite BMX for example) You can probably mount a side pull caliper style brake to the fork. There might be a mount in the brake bridge between the seat stays. You could have the cantilever studs moved by someone with competent brazing/welding skills, but a repaint would be necessary and you could probably buy whatever other frame there is to meet your needs for the cost involved.

You could use drum brakes. No reach problems there. Sturmey Archer has a cassette drum brake. They have front drum brakes. Choices in 650b rims is slim and expensive, though drum brakes would make for more choices rim wise (there are disc specific 650b rims). Consider 650a (iso 590) also. The don't have a lot of rim selection either, but there is the Sun CR-18 in that size now. Tire selection is also on the slim size.