Do you already have the belly mower..?? If you were planning to get new, Woods used to have mowers for the classic models, and the lift levers, and attaching pieces. It will NOT be cheap..!!
30- some years ago, I fixed up an A to sell with a belly mower. Bought the mower used, in good condition. The mounting kit, belt, and lift lever was right at $350.00 then. More than I paid for the mower, which was in great condition.
Just make sure you get a mower suited to your tractor. They make them in CW rotation, and CCW rotation. Some tractors belt pulley such as your B is mounted on the right. A buddy of mine's A-C CA is mounted on the left. Both turn CCW on the belt pulley, but being on different sides makes a belt turn the opposite direction, to an implement. Not so bad on the old flat belts, where you just flip them half over to correct direction.