I don’t know how hard-wearing your commercially available exterior paint finishes are, but I would imagine you have some that can take the tough weather conditions you experience in Ohio. What do people use to paint water tanks, bridges, buildings etc in your area? In the UK there have been some developments of paints using epoxies with glass micro-flakes which overlap as the paint dries, forming a very tough out er layer.
I would also look at protecting what remains of the original with a clear protective layer so that if anyone wants to examine the original in future there is a very distinguishable boundary between it and the repaint. The new paint may last longer if it is coated as well. How big is the painting? Would it be possible to have panels printed and mount them on the wall over the original instead of painting on it? Each panel could be sponsored by a local business or community member via a website. This was done very successfully with a JMW Turner painting The Blue Rigi
Hope this helps