Wrong cure for that problem
I wouldn't spend $75 on a toilet that can be replaced for about $125. And especially not one that old. Neither a joker valve nor rebuilding the toilet will cure holding tank runback. The only cures are:1. install a loop in the head discharge hose that's higher than waste can climb over at any angle of heel.2. Move the inlet fitting--and prob'ly the vent fitting too, 'cuz if waste is running back toward the head when you're heeled, it's prob'ly running out the vent too--on the tank away from the hull, to the side of the tank that's toward the centerline of the boat. Once a tank has been installed with the fittings in the wrong place, the loop is the only practical option. However, when installing or replacing a tank, always spec the fittings toward the centerline of the boat. Why? Because when you're heeled on one tack, waste flows to the opposite side of the tank...when you're heeled on the other tack, the inlet and vent lines are running uphill. Unfortunately, few if any boat builders and yards have figured this out yet.