Ryobi products are not designed to be repaired and to that extent there are few service parts available for them.
Down here that came second to Stiga for the longest delay.
Two years for a recoil and starter relay on a SP walk behind ( Stiga took 28 months to supply spindles )
As for your rings we need the bore size, piston size ring width depth of the ring groove in the piston, position of the ring pin ( top or bottom ) and the type of bore ( alloy, chrome plated , etc etc etc ).
From that we can find a ring that should fit from a brand of tool that has a proper parts supply system.