[Blue_Heron]

If it ain't broke, don't fix it. You won't improve the value of an original condition Potter hull by cutting into it.

I would not recommend raising the transom if it's solid. With that light 4cyl Johnnyrude on the back, 20" will be fine. If the deck isn't soft, it's because water hasn't gotten to the wood core. Make sure any holes drilled for fasteners, existing or new, are sealed so water stays out. Do this and your deck will last indefinitely. No worries about the stringers. It will probably need new wiring.

If you're concerned about the tank, you can remove the console and the deck hatch and inspect the tank. If it's not foamed in place, it's probably fine.

If it were mine, I would clean the boat, clean out the fuel tank, put some gas in it, get a new battery, and see if that old motor will start. Then I'd run it a while with the old motor to get used to the handling. Once you've used it, you'll have a better idea what you want to do to customize it.