That V brake adaptor looks like a possible solution to the rear braking issue on the 20...
I put on a new old saddle and my old Shimano 600 racing pedals (clips and straps) this am and the bike looks like like it lost 5 pounds... I am looking forward to taking the bike out later.
On my little spin home last night I estimated I was hitting 20 plus mph (90 + rpm spin)...Twenty's really go so much faster when you redistribute your weight evenly over the bike, get off the back wheel, and get much more aero.
With lighter wheels and faster tyres I expect to be able to hit 25 mph plus even though she will never be a lightweight (the frame and fork are too heavy)... my fixed folder was down 24 pounds before I reinstalled the factory fenders which is good for a bike that started life at 36 pounds.