If you've got a hobby shop near you, you can take the prop with you and compare.

If I were to guess, I'd start with a 9x6, and maybe grab a 9x5 and a 9x7 too. Props are only about $2 each. You can try the different props and see which one sounds most like the original in a test run.

To get more scientific you'd need a Whattmeter to test the Amp draw with the OEM prop, then test other props to find one that draws about the same current.