Brakes
Try this website:
http://www.francescobazzani.com/freni-e.html
I have a 210 mm Menani on my race bike, functionally identical to the better known Fontana, and slightly cheaper. My (racing) experience of big Japanese brakes (250 mm four shoe) is that they aren't as good as 210 mm Italian ones. On the road it probably wouldn't be noticeable. The 250 mm Fontana would probably stop a speeding locomotive.
If the standard brakes are to work efficiently, both levers should be parallel when both are hard on. Captain Vincent has a neat way to achieve this.
But. Vincent brakes are 7" drums, very resistant to fade ( a 7" brake with 2" wide linings) but lacking the diameter or design features ever to be very powerful. Rudges had 8" drums in 1934. Diameter for stopping power, width for fade resistance.......
Stiffening them up is worthwhile, classically by brazing triangular braces inside the steel plates or by buying "Lightning" plates.
If you stick with the balance beam, put an outrigger plate on. Going to twin cable levers removes the balance beam as a source of lost motion.I've tried both. Functionally it makes no difference. A given pull at the lever results in half that pull at each drum with both methods.
However even with ribbed drums, Lightning plates, turned linings and all, mine were fade resistant but not responsive, particularly in traffic. Discs are (IMHO) the best and cheapest option, a big four-shoe the next best. Both require the contrivance of a speedo drive. So I bought PV 2ls brakes: expensive (compared with making up a double disc system from a breakers), not as good as disc(s) or a big four-shoe, but perform very, very much better than I could ever get my standard brakes to perform. The bicycle still looks like a Vincent, and i didn't have to make my own arrangements for a speedo drive.