From the pictures, it looks like this bow has several inches of set, unless the stave/board was deflexed prior to making the bow. You don't know the history, and combined with the set in the pictures, you could consider it to be starting with damaged wood.
It is a bend in the handle bow, so you could probably pike it even shorter than the 68" recommended before the pictures which would increase the draw weight but more stress on the limbs. Cutting the tips down to 1/2" and re-tillering to a proper profile should reduce the hand shock. But if it is currently 50# @ 28 and you want 50# @ 26, this may not be achievable.
I would say if you want to do something with it, shorten to around 64", recut the limb profile to give you 1/2" at the tips, and re-tiller to give a proper tiller. It may be an experiment and blow up, but you said it was essentially useless as a bow before, so at the worst case, it breaks and you wasted your time. But you may end up with a decent shooter if the wood is okay. I think what everybody else is saying is that the amount of work you would need to do on unknown quality wood would be better spent on a fresh board. I say if you go into it with realistic expectations that it might break or might not result in your target of 50@26, then go for it and have fun.
