MunchkinMan said
here that Ship Enhancers can be carried unused.
I agree with that as all Items can be carried according to the rules. You can't use the Enhancer Items unless you have
Cheat!, though.
In this regard, they are like One-shot Items. You can place them on the table in a state where they don't apply any of their effects. I assume then that you can still play them at any time you could usually play that card, so once you have a ship, you could choose to attach the Ship Enhancer, but once it's attached it stays there.