Your search for "fleck 2510 service kits" revealed the following:

Adapter Coupling -15078-01

Adapter Coupling

Shaft, Rotor Drive, TwinFlo 100 / 8500 - 18776 *OBSOLETE*
As of 01/01/18 - this item is no longer in production, and is no longer
available from the manufacturer. Please call us at 888-426-5001 for...
Seal, Molded Outlet, TwinFlo 8500 - 19005 *OBSOLETE*
As of 01/01/18 - this item is no longer in production, and is no longer
available from the manufacturer. Please call us at 888-426-5001 for...
Gear, TwinFlo 8500 - 19062

Gear, TwinFlo 8500

Upper Piston Assembly, DOWNFLOW, TwinFlo 8500 - 60112