We struggled with bringing a smaller spare outboard
before we left cruising and just couldn't make it fit well. Haven't needed a spare yet after 5 years.
Some things to consider: if the 8hp is a 4-stroke, you have more limited storage
options because they can only lay on one side and are not forgiving of being tipped around much (heeling and pitching). I guess emptying the oil
helps that a bit. Also, outboards stored and not run turn into anchors pretty quickly on a cruising boat.