Replacing the transom on my aluminum 22' Starcraft Holiday

Hi guys, I'm a new member and learning my way around this site, so forgive me if I'm posting this in the wrong area. As stated in the title, I'm replacing my transom which is a first. I just got the outboard all disconnected and will pull it tomorrow. If anyone is familiar with this model (1990 Holiday), I was wondering how difficult it will be to get the old core out. Not sure if the factory glued it in. Also not sure if all the small philip head screws (about 25) will be difficult to get out since it's 22 years old. Also, not sure if the whole splash pan needs to come out or just the center panel. It looks like if drill out the riviets on the center panel it should come out without too much trouble but if the whole thing needs to come out it looks like extensive teardown. Also had a question of what to do with the extra holes from transducers and speed sensors that I'm no longer using (upgraded to new electronics). I have a 10" crack in the splash pan (outboard well) and have to get that welded, should I have him weld the old screw holes at same time before I put the new core in or just put screws in the holes with sealer? Thanks in advance for all your inputs. Bill D Clarkston, Mi.