It depends on what you mean by "custom". If the electric lift has been replaced with something other than a lift arm, then I am unsure and a picture would help a lot. If the PO replaced it with a lift arm, then a paddle or lift bracket is used to lift the thrower and that lift bar would slide into its hole. I have included two different pictures of these pieces that are currently for sale. The only difference is the type that doesn't have a channel in it requires you to remove the foot rest to install and the other one slides down over the footrest post and is secured with a long bolt--both will work for you.
http://www.ebay.com/itm/cub-cadet-na...item2a32d3cd32
http://www.ebay.com/itm/CUB-CADET-LI...item35cd13cede
BTW>That thrower is going to be very heavy to lift if you are unassisted. I recommend that you either install a hydraulic lift (several hundred dollars), an electric lift (expensive, too and slow--not a favored lift, but original to your tractor), or at least a spring assist (inexpensive and still effective--makes the load feel like about 1/3 to 1/2 of its original weight IMHO). I have also seen where some people hook up a winch to the front of their cub for this purpose, too.
Cub Cadet 123