[Zeke]

The framing looks light but as they did in '49. Not sure why you're getting sag, you've got purlins and I see a kick brace. There sure could be more bracing and vertical supports.

However, the rafters and purlins have taken a set. They won't move up nicely as you would like. I'd add necessary bracing with a little upwards pressure during the installation. The roof won't sag any more and at best, may correct itself a bit with a couple seasons of constant temp and humidity changes.

New comp roofs need plywood sheathing. To not add weight you would remove the board sheathing. But, if you add the bracing, no need to do that. You could table saw some sleepers that level the roof from rafter end to the ridge and nail the new ply sheathing over those. Probably take out most of the sag and wave.

If you do take off the boards, then it would be much easier to straighten out the roof with some sister rafters. You could do that with 2 x 4's. That would be the cleanest way.