Radio batteries are not very expensive - I'd just go and replace them.

Like Missileman said, clean the engine with carburetor cleaner. Put some in the cylinder through the plug hole, through the exhaust (remove the muffler and put it right on the piston) and through the carb. Let is soak over night and see if it starts to loosen up. If it’s still locked, add some more cleaner and let it soak for another day or so. After it loosens up, put some after run oil (automatic transmission fluid) in the carburetor, exhaust and plug hole, and flip it over to flush out the cleaner and make sure everything is well lubricated and protected.

Check the covering and plastic parts to make sure they have not started to dry rot from the heat. If you can, remove the fuel tank and check it closely. It may have castor oil residue and have to be cleaned out. Just to be safe, just replace the fuel lines – tubing is only about $1.00/ft.

Hogflyer