First off, plywood is sheathing, so, no worries there.

If it were me, I would rip out the existing down to the porch framing, add either 1/2" or 5/8" plywood, 50 pound tar paper and regular 30 year three tab shingles. Of course, all the appropriate flashing, etc.

This would both look nice and last a long time. And, not be very costly either!! :)

That's pretty much what I decided was the only reasonable thing to do. That along with painting the underside. I had actually thought about putting breadboard on the underside to clean it up and look good. Then I got looking at available things and found grooved pine sheeting. So now I'm wondering if I could use the grooved pine instead of the plywood with three tab roofing on top. Anyone comments?

Make certain that the rafters for the porch will be strong enough to carry the additional weight of heavier plywood, and the roof shingles as what you have now is more than likely 3 tab and most all roof's these days are Dimensional or Architectural type shingles (which will be heavier) as well as the potential weight of the beadboard.

The support rafters are 2x6's on 24" centers. The current roofing is roll roofing. Both the to thin original plywood and roofing are shot.

Definitely check the load calcs of your proposed roof before you move forward.