Yes it is a big big deal welding a crack in some places. A lot of shops won't even TOUCH it with the foam behind the crack.

Having a boat with .100 aluminum and a heavy motor over about a 115 HP is taking a risk you don't have to take. It is a mistake you can avoid. Xpress has been building these for a very long time and the bigger higher HP boats have thicker aluminum - at least in the transom. I am sure they do it for a reason.

Personally, if the boat had a 150 or bigger on .100 aluminum, I would not buy it. Why risk it? Find a manufacturer that uses a heavier structure.