How to seal rusted cleanout with broken bolts

Finally found the source of the smell in my basement. In a hole under the floor, accessible by removing a couple boards, I found an unsealed sewer cleanout. It had a rusted metal cover sitting on top of it held down by a pair of bricks. Looks like the original four bolts are snapped off and still in the holes in the flange. Any ideas on how to seal this? Preferably in a way I can open it in the future if needed. The white in the picture is the concrete foundation.

That looks like a old cast iron backwater valve with the flapper removed. How long have you been in your home - do you have problem with sewer stoppages- does your home sets lower that the city main sewer?

Definitely cast iron. We've been here five years now and the only problem we've ever had with the sewer was the smell downstairs. No evidence of any overflow in this pit and I can view water etc. flowing through the pipe without a problem.

I would replace that backwater valve or cut it out and install a clean out , but you should have a plumber out who is familiar with your area before making any repairs/replacement. Although you haven't had any problems there could be the potential for problems.