Will the boat plane with a single 50?
When you run a boat with twins, both engines are proper higher pitch--at least 2" and most likely 4"--so when you are trying to plane with a single 40 to 50, with the higher pitch, you are having a lot of slip, and lugging the engine.
With the correct prop, you can get about 12 mph, which will be planing in a light boat. This is based on talking to several owners who had single 50's correctly propped. I don't think this is the best way to go--but if you only want displacement speeds, then it would be fine.
Last night I was thinking about the boat--and I would really want to look very carefully for water in the core. Remember the 25 I bought in New Jersey had been left out in the open several winters, and the cockpit floor (not boat bottom) core had water intrusion around the hatches, and had been broken thru by someone jumping on it from the gunnel. The moisture gets into the core, around any screw holes, then with the boat open to the weather, snow and ice form, there is freeze and thaw cycles, and this increases the rate of delimitation.
It would be worth having someone with a moisture meter, as well as sounding out the boat, or using I R (Calibrated Camera, but could be done with the I phone App.) to determine water intrusion into the hull bottom.