Project just got a whole lot bigger

I am restoring my 1979 Monark Mcfast and took the top cap off today. The transom looked great on top and the motor had been on very solid. I noticed a little bit of brown water dripping from where i removed a screw on the bottom of the transom that was holding a transducer on. I enlarged the hole a bit and stuck a pick in it. The pick could go through the plywood with no problem and the material was dark brown and very rotton. I drilled a few more holes and found that the rot goes about 4-5 inches up from the bottom of the transom and then the plywood is wet but still strong up till the bottom motor bolts. I assume this will need to be replaced. Should I replace the whole thing or just the bottom part? And would a pourable transom be okay for my project? or is wood better? If I do decide to replace the whole thing with wood, do I just get the old wood out and slide the new transom down or do I have to tear off the fiberglass too?

If you go with marine ply for a transom it would be easier to remove the inner skin. Then you would install the transom plywood and then reglass the inside and make a new skin.
If you chose to poarable method the leaving the inner skin would be an advantage.
I don't think it would be possible to ever replace the bottom only. There are some ways to replace rotted top portions with the poarable products.

I'm in the same boat with transom. the seacast looks easier if you don't have any other major fiberglass work that needs to be done. Looks like some of these other posts have had success with it. Cost about $500. I'll be replacing my transom and who knows what else with wood and glass, probably still going to spend 500 but will be fixing more things.

im replacing the transom in mine with seacast. its just the top of mine that is rotted about 1 inch down from the top bolt holes ,but i dont want to chance it being bad somewhere i cant tell