I am guessing you have the 30 hp 3 cylinder two stroke.
GREAT MOTOR!
If it is a long shaft you may have a problem mounting on the 24 inch transom.....too high for the stock props.
The aluminum stock props go up to 14 pitch...... the boat will run low to mid 40's
Difference between the 12 and 14 pitch is acceleration.....very slow with a 14.
These motors can turn over 6000 so in the end there is not much difference top speed wise with the bigger pitch...the motor just turns more rpm with the smaller pitch.
I paid less than $80.00 Canadian on ebay for the aluminum props......stainless is WAY more costly....$200 to $450.
I would run the set back you got with the boat.
LISTEN TO THIS 30 hp Yamaha run on a tunnel!!
https://www.youtube.com/watch?v=IDg45ezSvZE