First you need to understand that the glue joints in all inflatables will go bad with time (Read, turns grainy).Secondly, running them underinflated is bad bad (that's not a typo). So if you have been doing that, I'd expect the lamination of the transom to the pontoons is the leaky spot.
The weight and hp of the motor
is fine. Don't beef up the transom. it's the transom to chamber glue point that is weak. The transom or floor is probably coming loose and letting water in. If the glue is letting go, there isn't much life left in the boat
. Seal it up and enjoy it. But pump
it up. You'll be much happier.
And any real glue work
you want to do must be done with the proper glue. If your boat is PVC, use PVC glue. If it's hypalon, Perpare to $pend more on the glue to do it right.
One of the many inflatable
boats I have bought and sold was an Achilles that was in brand new condition, except that it had been stored for years uninflated. Glued seams letting go. Wasn't worth fixing.