I had a ~6 gallon tank on my Com-Pac 23D (w/a Universal M2-12, IIRC), stainless and custom-made to fit under what serves as the laz/seat on those boats. It was that capacity because that was the amount of space available there. To run it dry would have taken 24 hours or more of non-stop motoring, as it had a 1/4gal/hour burn at hull speed, so you should be able to get by with a 2-4 gallon tank plenty fine. I'd go with a small tank like that under the cockpit floor like Bisquit has, and design it to be pretty easily removable for cleaning/inspecting. Shouldn't be hard to do, with that small of a volume. You can use an outboard engine bulb/line to transfer from a jerry tank sitting on the cockpit seat into your main tank with little drippage/mess, and keep the inlet inside one of the cockpit lockers for simplicity. Thanks for taking care of the old gal!