RE: Rehabilitating Cox engines
GCB has that right. The fuel will get them loose, afterwards you need to oil them up, a couple drops into the cylinder/piston port, and into the intake , and then pull it through the engine by prop flipping a few times. If a reedy, a drop on the intake, then flip her over to pull the oil into the reed.
If they are very gummed up, I would tear the motor down for piece by piece cleaning.
To store, Ziploc them.
Finally, when using the fuel to loosen, avoid direct skin contact as much as you can. Wipe the fuel off your hands. Fuel is a bad thing to play with, treat it with care.