The numbers on your sticker are the gearcase reduction. If you look at the prop you'll find a long number that starts with 48. The last 2 digits of that will be the pitch.
Ebay's actually pretty good for cheap props-just get a standard Quicksilver one.
A 50 will push an sr4 so fast there's no point bothering with flash stainless props and so on-you'll never actually make the top speed for more than a couple of seconds before you have to back off. Even a small ripple will cause brown trouser moments
To see what they go like with that prop have a look
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