That old big twin motor should be pin drive. Those old pin drive props will be hard to find in the pitch he will want.
However, If he can find a modular hub that fits his smooth prop shaft, with a brass hub that protrudes enough to drill for the drive pin, he should be able to find a set of props that may fit.
The modular hub for my '98 Johnny 150HPV6 props does protrude enough to fit a pin drive, and has a stainless steel cast cover to dress it up. Clearly the prop from my motor sill not fit that big twin, but if the prophubs for a smaller Johnny of the same year have the same protruding brass hub with the right diameter hole, it could work.