Several concerns...
If you mean by "chimney stack" the chimney extension beyond the roof line, then, above all don't use the fireplace. Additionally, if there is sufficient chimney above the roof line, you need to check if it is lined. Should be clay or stainless steel liner. Next, the smoke ledge just into the fireplace should be checked for security. If the fireplace hasn't been cleaned for many years, the chance of a chimney fire from built up creosote is high. Such a fire in an old, unlined chimney would probably result in the house burning down. Have it checked by competent professionals before using...