Any mower deck up till they went to the wide frames will fit.You will need the correct undercarrige and belt though.This can get real complicated.
You can take your pin on style mule drive,switch it to a QA style and use decks that were made for a 104-147,38" -48".Do you have a copy of the mower deck index CPE-2? It will help you out.Also a manual on installing one on your 123 is availible in the manuals section.
Here are some pix of the types that will fit.
Good luck and keep us posted on your progress