This is a "chicken hopper" tank setup and it is the best you can do for a Profile fuel system. The big tank is not uniflowed, it is to be completely sealed (with no leaks at all!) and connected by those two tubes, the feed and return lines, to the little tank. The little tank is uniflowed, with the inner location of the vent tube carefully chosen to be the virtual fuel level as far as the fuel feed to the engine is concerned. Assuming this worked once in the plane's first incarnation, you should be able to make it work again. The advantage of this setup is that the fuel pressure to the engine does not change as the fuel is consumed, right up to the point where the big tank goes empty. So you can have a completely consistent run through the whole flight.
The K&B 5.8, if it is in decent shape, can be quite powerful and should have more power than most of the modern non-racing engines you are likely to buy. The idle adjustments, on the other hand, might not be quite so easy on a sport engine. It is worth a try, at least on a test stand.
Pete