Need help with new prop for my motor

I have a 1971 Johnson 50E electric shift motor.

Runs great, but the prop that came on it seems to be too much pitch for my boat. It takes forever to get up on plane. I'm talking like 1/4 mile or more. Once it is on plane, you can here the RPMs go up and it acts fine. But it bogs like hell trying to get there.

It has a 13.25x17 prop on it now.

I called a local marina, and they wanted to know what gear case and how many splines the shaft was. I have no idea how to figure this out. Help?

I ran the turning point propeller wizard and it said I should put a 15 pitch prop on it.

However, I'm having a hard time finding a 13.25x15 prop.

Can I use a 13.75 or 13.5x15? Or will this not work?

Thanks!

Update:

found this part breakdown for my engine model

https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1971&hp=50&model=50ESL71S&manufacturer=Johnson&section=Gearcase

Which I cross referenced to this prop - https://www.ebay.com/itm/SOLAS-Prop-Johnson-Evinrude-13-5-15-Pitch-3-Blade-RH-2411-135-15-Amita-3/273061288728

For a 13.5x15 prop.

Worst case, it doesn't work and I send it back. Best case, I can sell my old prop and make most of my money back.

You will still have to know how many splines your shaft is no matter what prop you get. Simple answer to that is count them.