Probably not an answer you want to hear, but I don't think any of the newer model decks will work. They used the 50A/50C model up through the late 1990s on models like the 782, 1206, 1710, 1811, etc. At that point they changed styles to a completely new style tractor, often referred to as the "Cyclopse" Cub Cadets. These tractors had different subframes for the belly mounted mowers and I'm fairly certain that none of them will retrofit onto the older style tractors. One of the biggest changes is that the decks are taller and more rounded, and also a lot heavier with traction pads on the 54" decks where you could step to get on/off the tractor. I have an 1864 "Cyclopse" with a 54" deck, and an 1811 with a 50C deck, and I'm fairly certain that the 54 would not go under the 1811.
Here is a picture of a newer 54" deck off of a Cyclopse tractor. You can see how wide the front and rear hanger points are:
Here is a 50C like the one you have. As you can see the hanger brackets, especially the front ones are really narrow. Note, someone made extra reinforcing brackets between the wheels on this photo that are not stock... but they're interesting:
Good news though, there are a ton of 50A and 50C mower decks out there, so you should be able to find a replacement. Also, if you really wanted, they still make a replacement deck shell for a 50C mower, but I think they run about $400!!!!!
Bill