Something I've been thinking about:  I was wondering if propeller science/technology has increased to the point where installing a new and improved prop might give some additional and worthwhile "umph" to my old 70's model Johnson 9 horse motor?  And even if this were the case, do they make new/different props for old motors?  And is it "legal"?

Not much has been done in the way of props for the old sheer pin motors. There are only a few pitch sizes available and they are probably not moch different in design than the ones available then.

As for legal, yes you can change to any type prop you want.