How to seal these sewer clean outs?

A common approach is a rubber coupling (also called a "Fernco®" for the same reason adhesive bandages are called "Band-Aids®")

Counter-intuitively, the last time I read the instructions carefully the ones with a stainless steel reinforcement (sheilded) were not recommended for burial and the plain rubber ones (unsheilded) (with stainless steel clamps) were, but read the instructions carefully yourself to be sure.

General approach is to cut off the iron pipe and use a rubber fitting to transition to PVC. Then be sure to protect the PVC from sunlight (cover it in some way, or at the least paint it. Sunlight is not good for PVC pipe.) You may need to use a wire brush and/or grinder to get the outside of the iron pipe smooth enough for a good seal.

I have the exact same cast iron double cleanout, but the threads on mine are not as corroded as yours. In your case the simplest solution would be to clean up the threads as much as you feel like with a wire brush and then use an internal expanding plug which does not need threads.

A step further would be to get the right size tap and recut the threads to allow the PVC or ABS threaded plug to screw in. You can use silicone grease on the threads.

You could also investigate how hard it would be to remove the inner cast iron part with the threads. It appears that this is pressed in with a rubber seal. However, if you would do this, you had better be sure that you don't pull the cleanout out at the bottom.

EDIT

Google: Real-Tite® Reusable Expansion Cleanout Plugs/Covers