If the boat will accept a 15 HP, get that. However a 9.9 would do the job quite nicely. Read the capacity label and see if it tells you what the maximum size outboard is recommended.
You're probably correct. Old fuel that's gummed up from age. If the motor is running, I'd drain the existing gas and start with high octane fresh fuel and maybe even a fuel additive to clean things up. Even if this isn't the direct cause, it should be done at any rate.
Sounds like fuel starvation to me. Fuel injectors at 56 hours? Doesn't seem likely to me. However, I'm not a mechanic. I would check for a blocked fuel filter if it has one. Perhaps the fuel tank is starving for air? That's all I can offer to look for. Maybe a different dealer is best but...