Constructional Hearth

I'm new to DIY. I have removed an old surround and the fireback etc to open up the fireplace to fit a stove. The only problem is that the constructional hearth is crumbling and is in a state of poor repair generally. I think it is probably better to replace it. How do I go about it?
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Hard to say with information given. Usually jack hammer the beast out and replace. Is it cement, tile, marble etc...
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There is no decorative hearth in place. The constructional hearth is made out of a strange mixture. The top layer was cement. Below that was what appeared to be a very sandy cement. Below that there just appears to be rubble. I keep digging down and am now about 6 inches below the floor - there is no solid base. Amongst the rubble I am finding things like old floor nails, old bits of metal etc. The brick fireplace rests on a cement base. I am beginning to think that the crawl space in front of the hearth has been filled up with rubble and cemented over. Is that likely?
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