I think you need to make it difficult to remove items. It is a risk based game.
First level is to comply with your insurance so that should the worst happen you can re-build.
Second level is to make it difficult to sell when it has been taken
Third level is to remove the item.
I remove my boat instrument displays every time I leave the boat. ( they are multi-use so there is a good reason
). However I only lock the rudders in the cabin. You have to know the rudders can be easily removed to remove them, so if they aren't in sight... ( for a BC26)
If a person wants to remove an item they will and there is not much you can do to stop them hence complying with your insurance requirements.
To deter the theft of the engine further I have purchased from the Norfolk Constabulary a replacement for the cowling. This means another cowl will be required if the engine is stolen. The Constabulary has told me that they have never had an engine stolen when the replacement cowl is on ( this time last year).More effort as the real cowling needs putting on before you start it up.
I attach some details and how it looks on my boat.
Sea Simon - The company I am with requires the outboard to be locked to the boat in some manner. For those without bolts I have used a normal outboard lock.