If the piston is sticking at the bottom of the liner then there's a problem somewhere because it should be nice and free. If it's sticking at the TOP of the liner then that's normal but can feel bad with a car engine because there's very little leverage to turn it over. If your engine has the pull start then it can't be turned backwards because of the one way bearing used in the pull starter. One trick that might get the head button loose (so long as you can actually turn over the engine) is to loosen off the cooling head bolts about a half turn then quickly spin over the engine with the plug fitted (make sure the engine is lubricated with some fuel first). The compression will usually pop the button loose without shifting the liner.

The Mach 28 is a nice engine, so far my son has run about 20 litres through his with no signs of wear.