Hi, Joy. Floor furnaces are unusual in many parts of the country and I am sure that you are somewhere south of Tennessee. I'm sure that a poster from somewhere in Oklahoma, Baja Oklahoma (Texas) or Louisiana would know what you are talking about but there are only a few of those.
For the geographically challenged, floor furnaces are just a 2' x 4' or so hole in the floor with a gas burner at the bottom and a metal grate on the top. The hot grates are good at burning kids feet--ask me how I know? Since they are always built into a house with a full crawl space, it should be pretty easy to cap off the gas, remove the furnace and replace the joists by sistering or just nailing into the blocking around the hole. Then you will want to cut and install a piece of plywood for a subfloor that is the same thickness as the surrounding subfloor. A do-it-yourselfer with a little bit of experience should be able to handle this in about a day or less.
But now comes the tough part. Depending on your floor, you will probably have to have a flooring pro get some of the old flooring out of a closet or somewhere to use to match the rest of the flooring. This does require some skill and experience but a good flooring pro should be able to do this in less than a day, including putting something back where he got the old flooring.
Joy, I'll bet that you were happy to see the old floor furnace go since they were usually put in the hall and those midnight trips to riad the icebox could be real painful experiences also.