Time to take the head off, get a piece of wood that fits into the cylinder, and carefully tap away. I've done this 6 times in the last two years. Three times I was able to actually run the motor afterwards, once I dinged up the case and broke the piston, and the most recent time, on a moby, we got the piston out and haven't touched it in 2 months.
If it's that rusted up that you need to pound on it, don't expect too much out of the bottom end. If you are able to get things apart without damage and you have the skill set to rebuild a motor, just about any of these little motors can be rebuilt and back on the road.
I have 16 V1 motors and 14 bikes. I gave the worst 8 to my buddy last year to rebuild and he's got them all going. Several had sat out in the elements with no carb or head or cylinder for what looks like years. He revived the nastiest of stators.
Anything is possible with enough patience, knowledge, skill, and, of course, $$$.