Like Steve above, I never disassemble engines that have nothing wrong.

For a stuck contra-piston, I've used the following technique successfully many times.

If the contra-piston is simply "stuck", then the first problem is to "unstick" it, any way you can.
I do this by screwing it down slightly, i.e. actually increasing compression, just to unstick the c/p.
Then slacken off the compression screw "a lot" (unscrew it several turns).
Then put plenty of fuel in, and make sure you can still turn the engine over gently (if not, drain off some fuel!).
Then give the propeller a good whack!
This usually knocks the c/p back, no problem.
If not, repeat until it does.

IMO, there are two important points in this:
Increasing the compression means that you HAVE moved the c/p (i.e. it's no longer stuck).
The higher compression gives more force to knock the c/p well up the bore...

It works for me...