British/ISO BB threading and a 68mm wide BB shell open up a world of possibilities, including a modern sealed bearing cartridge BB. You should be able to find something suitable, although you may have to experiment a bit with spindle lengths. This is where a cooperative and sympathetic local bike shop is invaluable.
I have converted numerous bikes of the 1950s, 1960s, and early 1970s from cottered cranks to cotterless.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069