If you want to replace the transom from the inside then you'll need to cut back the sole and the stringers and liner all about a foot and a half or more to give you access to the inside of the transom. Then you can use a circular saw and cut the transom into strips down to the glass and use an air chisel to peel those strips away. Then you grind and clean up the inside areas of the transom and start by reinforcing the inner side of the transom and hull sides with cloth. Then then you have two options - wood or composite as the transom material and then glass the insides to hull and then reconnect the stringers to themselves and the transom. If you were careful you can reuse the sole (if it's in good shape) and glass it back in along with the liner pieces you cut out. If you find soaked foam in the stringers Plan B is not a pretty one but as Fitz said, if you cut everything back, you are sure of all the attachment points, i.e. stringer to transom.
If you don't want the destruction part of the inside, poured is ok but it is more complicated than it seems - look at Lars 170 rebuild from Sweden and pick his brain - he used a poured method successfully.
Replacing the transom from the outside is not easy to cleanly do unless you cut the transom outer skin off totally so you can cut the pieces of transom material to follow the inner transom, especially with the " side wings" of the transom (parts that extend above the transom on the sides). Then you also have to laminate the outer skin across and down the hull sides and hull bottom a foot to form a good connection to everything. and then fairing it all out - a ton of work.
Anyway, that's my $.02 - you have choices.
