To get a better variety of Inserts, you may be better off tearing out your existing wood unit, but only depending on how it is finished off. If it is a stone or brick, it will be costly to tear out. If it's just a Tile or some type of Wood surround, it would be a little easier and it could be redone alot easier. And hey, you can always sell the old unit to pay for the tear out.

As far as a stove fitting your existing opening on the Heatilator; the only one I am familiar with that would work is the Castile by Quadrafire.

You can look up your opening dimensions online (instead of getting in there to measure). Bring those measurments to a local dealer and see what will fit along with the options they can offer.