Hello William. Here are the classes of motors legally ran on your Skua.
4.2.2 The Early (Pre 1976) Classic class shall be restricted to two-blade propellers.
4.2.3 The Late (1976-1979) Classic class allows propellers with either two or three blades.
4.2.4 Only three-blade propellers are permitted for the Post-1979 and 8-hp motors.
The Early classics will be the slowest but progresses speed on down the list.
Maybe 15mph with an early classic. OMC motors only.
Maybe 18mph with Late classic. OMC motors only.
Maybe 21-23 with a post 6hp. OMC motors only.
Maybe 23-28 with an 8hp. Any make 8hp 2 or 4 stroke.
The Tohatsu 4 stroke 6hp runs maybe between a Late classic and a Post 79 6hp.
Hopes this helps
Cameron Walker