I think the point is he doesn't know where to start and is looking for recommendations. Being new here I'm going to answer with what I have and what I know. I have a 24 foot Northwoods with a 40hp OMC (2 cyl) running a 13p standard prop. It will spin at 5200-5300 WOT and give me 18 mph unloaded with clean logs. I am in process of rebuilding a 60 hp twin to replace the 40. That 60 came with a 17p prop. I put the 17p on the 40 just to see what would happen 3300 WOT at about 9 mph. That is bad for motors.
Using what I did, I would wager a 13p might be a better fit for your application. It may be possible to over rev with the 11p.
But it also depends on your intended use. A toon on a small lake will most likely just putt around without ever needing to get anywhere in a hurry so an 11p at part throttle would be fine. We toon on Lake Winnebago and can cover 80-90 miles a day, so I see more WOT stuff. We have a nice state park on the opposite end of the lake 27 miles away.
An 10p or 11p 4 blade high thrust prop would be better, but I don't know if they are available for a 65hp Johnson.
How shot is current prop, is hub spun, are blades not bent ? If with just nicks, paint deterioration you should test that prop as a starter on a wot spin as currently loaded, once achieved wot rpm is known can subtract that reading from max wot rpm range factory stated to know now many less pitch sizes need to go after.
To test any prop professionally, you need a tach, without it combo is prematurely dead. OB needs to run at max wot rpm range, once there can throttle at the rpm/speed you like most. The engine will surely thank you..
Happy Boating