Someone with welding experience could make a robust grate for that opening.

The amount of airflow you would need to dry out a cellar like that would mean opening up the entire floor.. why not just utilize a dehumidifier?

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Best of luck to you.

Wouldn't that grate allow rain water to drain directly into the room? Is there sufficient drainage in the room to deal with that?

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Not worried about pests or anything? I really want to see more pictures of this out of curiosity

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You can just buy one online...the hole is a square, it was probably cut to a standard size. Look for drainage grates.

Where are you located? First hunch for me is UK from the photo... I know nothing about how homes are built there, especially that old. I would think best action is to seal the walls and put a humidifier with a pump, but have you asked the neighbor what they've done?

If you’re serious about creating conditioned space you’ll be best off sealing the room, insulating the floor and walls and mechanically controlling temperature and humidity. That’s a lot of expense and maintenance. We hate to turn away from free space but attics and basements were never meant to be broadly used. The cool floor is collecting condensation. More ventilation would create still more water. Underground space requires a lot of modifications. That’s why garages and sheds do better.