1973 20' CC Transom woes

[pianewman]

The plot thickens.
I just found out that this boat had a breach in the hull, on the keel, at the bow, right where the hull cuts into the water. It appears that someone may have run it into a solid object, and then did a quick and dirty repair, painting over it. My dad bought the boat w/o seeing this repair.
So, one year ago, he opened up the place, dug out a good bit of wet wood (on the keel), filled it with fiberglas fabric and resin, and painted over it.

QUESTION: How can we know how much damaged wood is in the keel w/o opening it up, and (properly this time) grinding it until dry wood is found? As this potential problem is 15-16 feet from the transom, how can we know how extensive this rotted wood is?

Impossible question, I presume, but curious if the experienced folk here would consider this new issue to be a "deal breaker" for the transom rebuild, as there may be more structural damage to worry about.

FYI: boat was "re-decked" by a previous owner, and floor appears to be very solid.

I might as well ask, since I have your attention...how high is up?
Thanks again for input. I'm learning a lot here, very quickly, and so far, haven't needlessly spent $$$. Yeah, I know, it's only a matter of time...