A lot of this will come down to personal preference and the way the boat is being used. Generally speaking, yes, 140HP is enough for a small boat. Can having more HP hurt? Heck no. It really all comes down to how YOU will use your boat and your expectations/needs/wants. Take a test ride and see if it fits the bill for you.
How much heavier is a 140HP 4-stroke compared to a 150HP 2-stroke?