Pics always help when giving advice but...
Replacing the keel plate is not something I would not try first. I know some guys have had to replace theirs but first I would try repairing it with Marine Tex on the outside and Gluvit epoxy on the inside. Drill small stop holes at each end of the crack. Apply Marine Tex to the cleaned/sanded area and build it back to match the surface that was most likely eroded from repeated beaching. Marine Tex will last a very long time and is actually tougher to abrade than AL as long as it isn't sharply impacted that is.
If that is not possible/feasible due to significant damage then plating over it is another option.
On another note, Chieftain has no "o".
hoto:
