Based on the labeling, the media tank appears to be an uncommon size - 21” diameter x 54” height.
Pentair’s Structural USA brand 21” diameter media tanks are available in heights of 36” and 62”.
Clack’s 21” media tanks are 62” tall.
Therefore, replacing the tank will require an ~8” longer riser tube and reconfiguration of the inlet/outlet plumbing. The riser tube for the Fleck 2900 valve is 1-1/2” OD and should extend no more than 1/4" below the top of the tank.
The electromechanical twin-alternating Fleck 2900 valves are not typically equipped with “Hard Water By-Pass”, so you can’t just close the inlet/outlet valves on the leaking unit…the system will stop all water flow when the active unit goes into regeneration.
Should you decide to keep the softeners online and reduce the reconfiguration brain damage… You might want to contact Watergroup Companies, Inc. to find an exact replacement.
As Ghetto Wash suggested...replace the resin, too.