Go bigger...
IF there is nothing wrong with the fiberglass other then it's being pulled on, you need to back it with either thicker and longer/wider plywood, or remove that and replace it with two by fours.When I purchased my 22, the engine mount was only backed by fender washers! Needless to say, this wasn't acceptable. I removed the fiberglass wedge that was on the back and was stock as this was also causing problems. I modified the engine mount so that it would mount at the transom angle and still drop the engine in perpendicular to the waterline. I took a large peice of machined aluminum that was used by the previous owner as part of the engine mount and used this to back four of the six engine mount bolts. I then used a piece of two by four to back the other two. I can now hang off the back of the engine mount without worry, and I wiegh twice as much as the engine, so I think it's good.You are better off over building it like I did, then you won't have to worry about it happening again!Josh