Just spoke to my new mate at a bike shop in Sydney, you may have met him, Donny who worked at Cheeky Transport... He's been working on Bromptons for years and just told me S75s will work great, just need a black 3/32 chain with the same amount of teeth I've got on there now...
I’m assuming that’s the case, use existing bb and swap over cranks and chain ring. Speak to @hillbilly for more info..