The deck concept is something I'd lose.
This from someone who has submarined both a catamaran sailboat and a powered aluminum pontoon boat. The fun factor diminishes rapidly when the bow goes under and the propulsion keeps pushing it further.
Center of Gravity is important as SevenCubed mentioned. I'd think about replacing the center bench. There's a reason they aren't built with a deck, but with benches. CG stays consistent when everyone is sitting.
Both my submarine experiences were the result of too many people moving from one location to another while under power, it doesn't take much. I'd also pitch the older main motor. Rebuilding might seem good, until you're way out and nobody is around, and the motor won't fire. Or, the motor won't fire, you're drifting to the surf line and starting to consider how soon the "abandon boat" order goes out.
I've never "lost" a boat, but I've had some interesting times. Less interesting is probably better.