When you modify a boat, you ARE the engineer. The boat's original data plate, depending on your mods, may no longer be worth a thing....
My thought on what you are trying to do, and what it might take to get this '97 20' boat to pull tubes, would be try to get an idea of what this might cost. It CAN be done for sure. It WILL be expensive. Then compare this cost to what a boat already equipped to do what you want to do might be? You may find that your money ahead selling your boat and getting one that will do what you want...
If you are determined, rather than add a tube, maybe the better (more effective) plan would be to replace your existing tubes with larger diameter (25" minimum) tubes with lifting strakes. This will dramatically increase the boat's efficiency, and increase it's capability to pull tubes easily. Though it would doubtless be working at max capacity, depending on your boat's weight, your 50 might pull kids on a hull like this.
