It does sound like you've got a bad fit between the piston, ring, and cylinder wall.

Of the part you're asking about, about the only part available for the US Engines 25 cc engine is the piston ring. The 25 cc engine has been discontinued for about a year, and parts are becoming scarce for it.

Apparently, the manufacturer has changed hands again, and the 25 cc isn't in the new owner's plans.

Here's the part's stock number and list price:

USEG1515 330008 PISTON RING 25 $11.99

This can be ordered through any hobby retailer who works with Great Planes Model Distributors.