Ha!     you have my attention… Crosby sled ?     see mine ( I sold  )   and  then a copy of the one on the FB glassics  site … “Crosby Cruisette”  with  2 bunks,  wash basin, overhead water tank,  head (hand pump overboard)  flip up windows.. the works .

Mine is sitting in the sun in my silly neighbor’s yard junk pile 🙁 grrrrr !

Yes, salt water will cause damage.  Go on Facebook Marketplace of an oceanside community and search for old outboards.  Then do the same search in an area far from the ocean and surrounded by freshwater.  After looking and comparing the old outboards, you’ll see the effects of saltwater on the motors.  The metal will corrode away, you’ll get white deposits, and the paint will peel where the metal is corroding underneath.  Additionally, bolts could become harder to remove after exposure to saltwater.  So best keep your Evinrude in freshwater.

Also, I’m not familiar with that boat and it’s motor capacity, so I’m not sure if the following is related to your plan with the 50hp 4-stroke replacement or not.  I’ve been told that since older two stroke outboards are normally lighter than modern four stroke outboards, with boats built to hold 2-stroke outboards, it’s a good idea to size down the hp of motor being used when switching from a 2-stroke to a 4-stroke so there won’t be extra weight to damage the transom.

Hope this is helpful!