IDK , I put a Schlumpf Crank on mine. its a heavy internal gear 2 speed with its own BB
(doesn't even thread into the frames BB shell threads)

Campag a Square taper ? I have a Campag Race triple on my Road Bike Its using their 111 long cartridge BB.

AFAIK they designed the double to use the same spindle. (ie the Rt crank is different )

Campag Track* Crank instead? *track frames at the back wheel are narrow almost as narrow as Brompton is.

you can seek out Square taper Dura Ace stuff on EBay buy several lengths of spindle to find what works?

I have the removed 50t stock Brommy one , thew chainring is fixed to the crank arm ,
probably lighter than the new 130 bcd spider crank , I'd sell It .. w BB

but out side the US shipping is expensive,