Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Bummer!  

Since it only will be used occasionally (i assume?), i´d simply make a brass sleeve, and even a brass piston as well if the original is easily removed.
That way you´d swiftly have a running engine, that will last you for decades, and on top won´t rust internally, if a bit of moisture is present in there.