Ayuh,.... The "Right" prop is the one that will allow yer motor to rev to 4400/ 4600 rpms, at Wot,......
Any less than that is luggin' the motor, at All rpms, which causes detonation, which burns holes in pistons,.....
Ayuh,..... If I'm not mistaken, the OMC spline interchanges with the Merc splines,.....
I know my Land & Sea Torque Shift props will fit either of 'em,.....
Ayuh,..... It's probably 1/2" pipe,..... very common,......
The other possibility is 3/4" pipe,....
The Od is 'bout 1/4" larger than the actual size,......
Ayuh,..... I run 15W-40 diesel motor oil in, quite literally, Every motor, from heavy equipment to single cylinder lawn mowers, 'n everything in-between, includin' my boats,......
Ayuh,..... In a word,..... Nope,.....
It takes a boat load of special tools to rebuild an Alpha drive,......
That's why the SEI replacements are the best answer,.....
Either the cost of the tooling, or the cost of the labor, 'n parts for somebody else to do it,......
Just ain't worth...