Probably too late to help here, but I did seacast on an old boat of mine about 10 years ago. It was a fiberglass Champion bass boat so a little different, but same idea. Get absolutely as much of the old transom out as possible. I started with a drill bit the same width as my transom and chewed up and removed as much wood as I could that way. Be very careful not to go too deep and puncture the bottom. I did that on mine once, luckily with fiberglass I just glassed it from the inside before I poured in the seacast and then gel coated the outside to match as close as possible. I used an electric chainsaw and could lay it against the front and back of the inside of the transom and “shave” off the wood there after I had most of it out from drilling. Take your time with this part, the more wood you get out the better. Engine on a cherry picker so you can slide it out of the way to get everything. Make sure any screws or bolts into the transom are removed before you start also.
Once you have all the wood out, the seacast is easy. Cover your mounting holes with tape. I put dowels through mine and coated them (I want to say with vaseline, but I don’t remember for sure) so I could pull them out and not have to re-drill the mounting holes for my motor. Mix the seacast per the instructions and pour full. I put a heat lamp on mine as I did it in the winter and wanted to keep the temp up to assist with curing down inside the transom. The seacast itself will get very warm as it cures, make sure you have a well-ventilated spot to work. After it has fully cured (and don’t rush that part either), refinish the top and remount everything. Tedious work, but not overly difficult.