The plug is missing from the exit side of a you trap cleanout in the crawl space of the house I bought. It is plugged with a rag. Is this safe for sewer gas?

No get a plug, a rag just keeps birds from flying into it. It will do nothing to stop sewer gases.

You are lucky it's in the crawl space. In the house, you would have known rags don't work! PVC plugs are cheap.

I agree but you need to determine the type of pipe you have before you go plug shopping. ABS and PVC get welded with a glue and primer. Galvanized is threaded.

Depending upon the size and material go into a plumbing supply house and ask for as clean out cover, (see image). Good luck, Tom

Thanks for the input. The rag was just a temporary fix. The previous pwner must have tried to remove the old plug possibly with the wrong tool and broke the insert for the square wrench leaving a hole in top. The rest of the plug is still intack but I will probably have to use a hammer and chisel to remove it to replace it. I hope it is not glued in.

It is PVC

The cover screws in. It isn't glued. Good luck, Tom