My factory tikit has a 74" top gear.
Under budget, a new Dahon Curve aftermarket fitted with that Sturmey-Archer XRF8W and geared 54x20 would yield
Attachment 549683
I do not think you can put a 20 t sprocket on a Curve. Long time since I played with the gears on mine so I should be careful to trust my memory but I remember wery clearly that therer was a limited space at the back.