First things first, an "insert" is something you place inside another fireplace. Both of your fireplaces are Zero Clearance or prefab fireplaces. I've looked at the manuals for both and neither will accept an insert (the majority of ZC fireplaces specifically say that you cannot put inserts into them, some installers and re-sellers will try to tell you otherwise, don't listen to them). Most "inserts" are designed to install into an older open masonry fireplace, although some ZC fireplaces say you can put inserts into them.
So you have a few choices. You can line either chimney and install a wood stove just outside of the fireplace(s) and connect the wood stove to the liner, provided the liner and stove speicify you can line the type of chimney in either case. The double wall temco chimney may be tough, you usually can't line those.
Or, you can tear out either one and replace them with a new ZC efficient (EPA) fireplace. I did the latter. I had your exact same Majestic there (in 42" version, not sure which yours is), I
tore it out and replaced it with a
HeatnGlo North Star.
If your going to replace them, one major area of interest is the chimney. All except one brand of ZC fireplace use proprietary chimney (I know, it's stupid, but being "built in", that's the way they are). The Temco chimney is not usable with any other brand of ZC fireplace, so if you replace that you will have to replace the chimney. The majestic probably uses a triple wall "majestic S8" chimney, the plate in the inside (inside toward front, on right towards the top IIRC) should tell you exactly which chimney was used. The older ones could take a double or triple wall, the newer ones specified only the triple wall.
If you have the triple wall AND enough height and width to play with, and you have some patience or can find the right installer, and if you can get behind there, you can carefully remove the chimney from the majestic and re-use it for a new Vermont Castings (they own Majestic) (broken link removed to http://vermontcastings.com/sequoia.asp)(EPA, high efficiency) which specify that same S8 chimney. It's a tricky thing to get that pipe off the fireplace, but if you can do it, it'll save you a $1000 or so in chimney pipe costs. I didn't have enough height to a load bearing header to use the Sequioa, so I choose a completely different unit and had to install the chimney specified with that unit.
Replacement in either case is fairly straight forward, if not involved. The first thing before you do anything to to know what the dimensions are of the old unit and what the requirments are for the new and what type of dimesions are in the space currently being occupied by the current ZC.
Here's a general
Temco manual and the (broken link removed). Not sure if they are the right ones, if not, then look them up yourself. Height, width, depth, chimney diameter...etc. See if you can even get a new unit into the space in which the old is occupying. Keep in mind things like if it's on a non-load bearing wall you may be able to adjust the header height and width, if it's a load bearing wall, your stuck with the current space...etc..things like that.
One you have that down and have picked a new unit, get any material covering the fireplace front off it, cut the chimney collar off from the front or from getting in behind in the wall or chase, and remove the old chimney pipe, then install the new one according to it's instructions. You'll need access into the cavity in which the fireplace chimney is installed. Mine was in a chase, so I cut an access hole in the chase wall from the outside and worked from there. I had to line my chase with insulation and type-x drywall, as it was not previously..things like that..keep in mind, you may have to modify to meet the spec of the new unit.