Is a stove chimney header sufficient for a fireplace?

[meanwhileinhell]

Hi, we moved into a property that had a wood burning stove fitted in the living room. It turned out that the stove was ridiculously dangerous so we removed it, to replace it with a cast iron fireplace. Initially we only wanted it to be decorative, but are wondering now if we can use it. My question is, will the header that was used for the stove supply sufficient draw for the open fire? (The header is just a concrete slab with a hole for the flue from the stove in the middle, but I'm sure I don't need to say that)

Using some smoke pellets, it doesn't look to have a particularly strong draw, but would, say, sweeping the chimney help?. Also, we definitely have birds nesting in one of our chimneys, but I'm not sure if its the one for the living room, so if it is, I guess this would dramatically affect the draw.

I just want to know if the header will be sufficient before we go getting the chimney looked at.

Thanks.