now, if only someone made a modern version of the R20...
I would change...
1-1/8 fork steerer, slightly shorter head tube, 406mm wheels with plenty of clearance for fat tires and fenders, 135mm spaced rear, disc or canti bosses, 68mm BB shell and replaceable RD hanger, updated bosses for racks and fenders and kick stand mount.
but retain...
all steel, same latch, same 28.6mm seat post size and same silhouette.