I think what Charlie is referring to is a Furnco adapter:
http://www.homedepot.com/b/Fernco/N-5yc1vZ4hs . I know its rubber, but whatever you use needs to be watertight.
If that existing line is caste iron, which was the standard around here before PVC, I'd consider replacing it all the way to the house plumbing. Over time, caste iron will accrete, or grow inward, as it accumulates rust and debris until you finally have clogging issues.
The oversized inlet hole in the tank can be filled with hydraulic cement, but you'll first have to get the contents pumped by someone like Roto-Rooter, and get the roots out too. Unless you are looking forward to doing repair work on your septic yourself, it might be a good idea to just hire a septic contractor to fix it all. A contractor can also fill you in on whether or not you'll need a repair permit issued by the local health department.
