Re: 25 HP enough?
Way back when, before I told God where to put the dirt, I had a 20 on a 13 footer similar to yours. It got the boat up on plane and it probably did 18-20 MPH (guesstimate--no speedometer) with just me in it and sitting back near the transom. So if getting to the fishing holes fast is not a concern, and if you are not carrying a lot of gear or weight, then a 25 might just do it. 4 cycle engines are heavier and have a different torque curve so I think in this application where a 25 is really marginal, a 2 cycle engine would be a better choice.
Bigger--up to the max rated --is usually better, so if money will allow it, buy a 35 or 45. You can always throttle back a larger engine--you can't get more than the smaller engine is rated for.
Actually---I have a 125 that would be just right for that boat--LOL The 140 might be a little much--Just kidding.
What year is that boat? it looks to be in pristeen (Oops! women's product) pristine condition. Did you restore it? Also, I noticed no cleats in the bow or on the deck--They come in handy for anchoring.