I am going to try to post a picture of my issue, but description is as follows:
I have a lead trap below my toilet that goes to brass with what I think is a wiped joint from my internet searching, then to cast iron drain. I think I blew out a piece of the lead pipe a while ago while snaking the tub (not sure how else it could've been so damaged before the joint). So I started cutting out the bad section thinking I'd try to fit a Fernco in there. My questions:
1) the outer diameter of the lead and brass (?) is about 4.5 inches. Should I try to remove the rest of the lead pipe and joint wiping going to the brass and then try to squeeze a Fernco in there going from the remaining lead to the brass? Both sides are pretty rigid. If I should do that, what size Fernco should I use? The 4" Fernco measures about 4.5" inner diameter, and I haven't been able to find a 5" X 5" one. Should I stretch the 4" one?

2) Or should I remove the toilet, all the lead, and the brass, and try to go to the cast iron with all pvc?

There is not a lot of room to work in the space, as you can see if the picture uploads correctly.

Also, another concern I am having is that if I call a plumber to do the job I think it is no longer up to code, because there is no vent pipe connection in sight.

I really appreciate any advice from the experienced folks on here!

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You are going to have to open the area up now and replace back to the cast with either cast or pvc. There isn't a good fix that you can perform on lead as it is too soft to repair with any real chance of success, especially over a living space.

yeah, pull the toilet and replace the flange etc all the way back.
i have had good success melting the lead wiped joint off the brass ferrule and connecting a fernco right to the brass.

Thanks for the replies! Would it be better to get rid of the brass too, in order to avoid the Fernco all together? If it is all the same I would like to try to leave the brass because it looks like it's going to be difficult to get out whatever was smeared into the threads of the cast iron to help seal it (it is hard). Thanks again for the info. -Mike

That's lead, and it's a bit of a P.I.T.A. with lots of room to work. If you were to remove it, then you're left with sealing the pvc in a cast hub, and that usually involves a rubber donut. I've found getting the donut in correctly to be another PITA. Use the banded coupling.