Just for the fun of it, add some weight in the front of the hull, about 5lbs. Try to keep the weight in the center. Run the boat and see if it makes any difference. Looking at the close up pics, it looks like there is a lot of weight on the transom. If too much weight on transome and the bow is lite, this will cause the hull to sink to much in the rear. For the extra weight it takes extra displacement to float the boat, the less displacement the greater chance of rolling. The 5lbs of added weight to the front is just a quick way of testing, if it helps in the problem, then move the engine and cage forward. The 5lbs is just a rough weight, testing with different weights may be needed.
Hopefuly this will remove some of the leverage the engine has gained from the displacement.
Off subject just a little, reading back over the post here, it proves that the airboats are not just fun boats, there is a lot too them, that many do not realize. This statement comes from hearing in the past, that airboats are just fun boats.