More then likely that 'calcified stuff' is the remains of the anode rod.
Anode rod material is wrapped around a steel rod that is welded to drain plug hex head.
Not the first time they have separated when trying to remove a drain plug
BUT it is a 43 year old water heater.
Time/effort not worth it.
That G602 should have a installation opening 12 3/4" H x 12 3/4" W x 20 3/4" D
New Suburban 6 gallon water heater features 12000 BTU's burner,
and is 12-11/16" H x 12-11/16" W x 19-3/16" D.
(All same size...gas only or gas/electric)
Looks like a fit.
(Atwood is completely different so no swapping brands w/o modifications----6 gallon 12 1/2" H x 16" W x 18" D)
