Clean out repair idea?

I cleaned out this clogged up 4" cast iron drain going out 15' to a septic tank and when I tried to screw in the brass Clean out plug the darn thing cracked on me. The plug is one of those thin ones that are only about 1/4"-1/2" thick and I only screwed it in maybe 1.5-2 turns and it cracked the cast iron on the left side of the picture. It was late and I was 30 minutes out of town so for now I just smears some silicone on the crack and I got it to seal but I would like to go back and do a more permanent fix on that cracked clean out. The Wye is all the way up to and little bit in the wall I think so it's not a simple thing to replace the Wye.

Does anyone here have a good idea on how better to seal/repair the clean out ?

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