Shower missing vent, or circuit venting?
I live in a house that's up on pilings, built in 1970, with leaded-in cast iron piping. As part of a bathroom remodel, I would like to move the toilet over a few inches. It's plumbed directly into the main horizontal drain, so it seems like it'd be a major pain to just replace that section. And since the pipes are getting close to 50 years old, probably better to just replace it all with PVC. But if I'm replacing it, I need to do so to code.
This is the layout, ignoring traps (they are there :) ). I'm not planning on changing the location of the fixtures at all - just moving the toilet over a bit.
The main drain is 4" pipe, and the main stack is 3".
The part that concerns me is the shower since it doesn't have a vent of it's own, but I'm not sure if it falls under allowed circuit venting (I admit I don't really understand circuit venting yet, but this layout seems to match that closer than anything else I've found). Or maybe since the shower was a bathtub, they got credit for the tub's overflow? Regardless, it's just going to be a shower now.
If this layout ok? If not, how can I make it right? I would prefer not to have to run another vent through the roof, if possible. I'm in North Carolina, which is using 2003 IPC with amendments.