Vent Collar Broken on Cast 3.5"T How to fix?

Hello
Im in the middle of a bathroom remodel and I found the collar on the 3.5 in main drain that the Vent pipe threads into has broken off. I have no idea how I should fix this.
The 3.5 in cast iron drain comes through the floor to a “T†1.5 in galvanized for the sink branches to the left and above that is the 1.5in straight up vent pipe. The collar on the Cast T where the Vent pip threads into has broken off.
What is your recommendation for a fix. Water never really flows over this joint except for rain water down the vent or if the main drain were to clog. My first though was lots of JB Weld. Any help would be greatly appreciated.

If you clean out the leaded joint, you can probably find a proper sized Fernco donut to make the transition. If you can't find one for the galvanized pipe coming in, you could change a section out to maybe pvc. You'd want to make sure that the vertical pipe was supported properly, otherwise it could gradually sink down through the fernco until it hit a bend.

You may just want to relead it.