Crack in transom, port side only. What do you think?

[jeff6645]

After sever days of fishing very rough seas I noticed that a couple of cracks has developed on the port side of my transom (red lines). If I grab the foot of the motor and push up and down with all my weight I can get the crack to open a little and close. I figured the transom was shot... I starting drilling some test holes to see how mushy the wood is... well the wood comes out nice and clean everywhere except on the port side around the cracked area and about 8" below the crack and even there is it not totally mushy just dark and damp. Should I just plan on replacing the whole transom or just redo the area where there is rot? Can you use Seacast to do a partial repair? I would like to get back on that water as quick as possible... maybe do a complete replacement this winter and fix the cutout that previous owner made to mount a short shaft engine. Thanks in advance for your input!!

Yamaha 250 two stroke long shaft on a bracket.
New transom 12 years ago - previous owner.
1974 23' Seacraft