14 foot lowe line aluminum jon transom question.

[eubbie]

Hey everyone I could use your help. I recently bought the above boat for 3 hundred from a guy I work with. It was obvious that he had rebuilt the transom at some point. This is my first boat and I'm very new to all this so bear with me if you can. This particular boat has about 18 inch sides from floor to top of gunnels. The wood it looks like he's used to rebuild the transom is a single 2 x 8, that runs horizontally across the top of the transom. It also looks like he has put a horizontal brace across the back of the transom to use to support the lower end of the 2 x 8. Is this the correct way of replacing the transom, or should the wood go all the way down the stern side of the boat to the floor? If it is incorrect, will this fix work, or do I need to redo it. There also appears to be a crack on the right side of the transom which is about an inch long, at first it looked like he had cut it to bend the aulminum up to put the 2 X 8 in there, then bent it back down, but now I know thats not the case. The crack is just left of the corner brace that runs from the stern to the right gunnell, as your looking toward the bow of the boat. I have some fairly heavy aluminum plating ( 1/4 inch ) that I can use for extra bracing. I really wanna fix this right though. The transom appears very sturdy, but I want to be sure. I plan on putting a 20 hp mercury on it, and it weighs probably around 80 lbs i guess? Also, the coast guard rating is gone off of this boat, would a 20 horse mercury be too much for this boat? Its a 14 foot lowe, 36 inches wide at the floor. Any help you all could give me would surely be appreciated.