RE: OS 48 4-stroke
Most parts are still available for this engine. You'll likely need a new ring and possibly a piston. You'd want to lightly scuff the inside of the cylinder liner with 600 grit sandpaper before assembling the egnine.
OSMG7758 45806000 PISTON PIN FS48SUR $6.49
OSMG7830 45803400 PISTON RING FS48SUR $22.99
These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.