What ever the core is currently made of is basically irrelevant if it needs replacing...
The only way to pour a 'Sea-Cast' type transom would be to somehow dig out the old transom and create a "mold" to pour the liquid transom in...
The way the boat yards around here would do one like that is to cut off the outer skin, remove the bad, install the new, find a way to connect it to the stringers [ in most cases, some interior destruction and reconstruction is necessary to properly re-attach the transom], re-attach the outer skin, repair the gel finish as needed and be done with it...
The only caveat is whether or not the stringers are in good shape or not...if not then either selective surgery is done, if possible, if not possible, cap removal is indicated...huge money!
The first method described above, done by a good boat shop is the least expensive, obviously and relatively...depending on the labor involved and the size of the boat [ for charges relating to the dry dock/pulling and re-launching the boat], 3-5 grand is normal...not including any electrical/mechanical issues...
Don't expect much more than a "If it Floats and don't leak" warranty, though...
If you can do it yourself, you would obviously save a ton of cash in labor, but a ton of sweat equity would definitely be in your future...
Again, just to reiterate, if the transom is rotten, do not be surprised if some or all of the substructure is, too...unless of course it was fabricated originally out of materials other than fiberglass encased wood...
Best of luck!
GT1M
