Thats a very nice looking bike, I wouldn't want to frankenbeemer it.
However, the dual disk upgrade is a very desireable thing, the single
disk just doesn't always cut it. My R60 with the front drum will out brake
my R90 single disk any day. I'm currently working on the dual disk conversion
myself.
An easy option for you is to swap the complete front end from the S bike.
Just drop it at the steering stem and move the whole assembly to your R90.
( I would put in new steering head bearings )
You will also need to swap the master cylinder. A new set of stainless steel
brake lines if you don't already have them and you're set.

As for a collector bike, I would seriously consider fixing the S bike,
If you have a little patience, parts can be found for reasonable prices
on E-bay, Just do a little research and don't get in to a bidding war.