Welcome aboard @Norman . That's a boat worth wrenching on for sure. Looks like it's in fairly good condition.
I have no idea what it is or what year it's from but others will have an idea, I'm sure.
Thanks for posting and we hope to see your project as you progress.
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.