Doing it this way would be tempting fate.
Even small variations from the original would make the job very difficult.
Initially I would suggest looking at the problem very carefully and then giving the matter lengthy thought.
This normally involves hanging upside down in the boat with good lights and suitable mirrors. A camera
or video recorder together with sundry tape measures and suitable sized rulers and other measuring euipment would be useful. An assistant is necessary, particulary one who can take good notes.
Then if it were my boat I would suspend the boat in a cradle
so there was no weight on the keel and then cut inspection
hatches as needed so that the inside of the keel can be seen/worked on as necessary.
Then I would make up my mind on what to do.
It may be that the entire keel needs to be replaced but it may also be that reinforcing, patches and the necessary persuasion will do a job that is stronger than the original design.
It sounds like the owner wants a quick cheap
job when what should be done is a slow expensive one.