Opening up fireplace

[Danielscrable]

Hi all, iv been in the process of opening up a middle floor fire place on a late victorian house and eventually installing a wood burner. when getting level to the constructional hearth it is non existent. It appears that in the past the hearth has been removed to install an old kitchen extraction hood ( not there or used anymore).
I have cleared away down to solid stone my question is what supports the fire in an old chimney would it be OK to just pour and tie in a new concrete hearth?
Thanks