Damp fireplace

[Joleneh]

There’s some damp appearing on the side of this fireplace, now having removed the electric fire and surround, I can see the floor is crumbling and is damp
Please correct me if I’m wrong with the following plan,
Dig it all out, check and replace joists as needed
Do I need to create a vent in the closed fire place?
Thanks

 

[a twig]

A closed chimney should be ventilated yes

 

[a twig]

Assuming it is ground floor - that hearth might go all the way done to the ground, if you’re definitely not planning on ever using that fireplace you could dig that out and sling joists and floorboards over the hole