Wholly old timers !!! Thos Mac 80s were clones of an ECHO SRM-200 engine from back in the mid 80's early 90's era.
Ignitions in those are RPM limited at @ 11K if memory is correct. Bore is 32mm stroke 26mm, displacement 21cc's

Hate to burst your bubble, but engines are not worth a hoot as a boat engine, without being able to spin them up to higher rpm you can't get any HP from them ... been there & attempted that .... 20 years ago !!

Transfer ports are really small and casting allows very little opening up larger. 2 bolt cylinder and if i remember the 80's correctly, piston was chrome plated and cylinder bare aluminum. The 95's were aluminum piston & chrome bore.

Sorry to sound so negative, but these engines and the trimmers they were on were part of the start of disposable power equipment and the final nail in the coffin for Mc Culloch corp.