According to Messick's part # K5639-36740 fits the 54" mower deck of a Z125SKH. About $90.
Doesn't show dimensions though. Give them a call, they're pretty helpful. Or ask your local dealer.
Edit
Searched Messick's again and came up with K3021-13560 so now I'm confused.
If it's breaking you have other issues. Fix those issues. Mine still has the original belts, close to 1800 hours on it and 1600 of that from the original owner who must have used it every single day. She didn't take very good care of it either.