I don't mean to upset you,, but I don't consider that bike to be a good candidate for motorizing. This is MY opinion, now.
Yes, S-6 is a distinctive Schwinn rim design. Always buy tires designated for S-6 bikes to go on those rims. Even if you tried to rebuild the wheel with thicker spokes, you still are limited in choices of tires to use, which are going to be the skinny tires. Cruiser type bikes seem to be the best candidates. to motorize. Plus, who wants to go very fast on such skinny tires ?? NOT me !
You can continue to restore as original & you will have a super nice bike for normal bike riding.