Water heater vs tankless water heater in 2000 Adventurer

A long time ago, back in the 90's, they used to sell just the tank as a replacement. You had to take everything apart, remove the leaking tank, and replace it with the new tank. It worked for me. You might check with reputable suppliers to see if they have replacement tanks. As I remember, it saved me quite a lot of money over replacing the complete water heater with a new unit.
Good Luck,
Journey Indiana