Is there scope for a short solid extension piece to be fitted to the splined shaft end, at the risk of possible balance issues to resolve. Made so the spline end of the adapter has a short solid section before the keyed end section, so they don't coincide creating hollow weakness.
My rough backyarder solution: cut down and turn down an old prop to get the spline needed to mate the shaft. Weld some solid bar to the end, and turn it down (clocked up to the spline) to the diameter and length needed for the prop. Mill the keyway as standard to fit the prop. Scotch key the extension piece to the shaft's splined end in a likely permanent fashion. Drilling and tapping hardened spline and tough shaft may be difficult; weld would be permanent and could fail. Also added prop distance from skeg support is bad. Modifying or heating the shaft will weaken it anyway. My solution may work for an old put put boat, but not a modern powerhouse.
Easier to get a new or S/H splined prop that fits.
