This is one of a couple of steam toys that I have sectioned for demonstration purposes. This one is (was) the engine unit from a Wilesco D16. (see here for a whole model)
Only the cylinder and slide valve have been cut open to enable viewers to see what's inside. The workings of the valve linkage are visible without sectioning.
The sectioning was done mainly with a junior hacksaw, finishing the cut surfaces with a needle file and emery cloth. The thin sheet of the cylinder cover was cut with special shears. The piston and slide valve are, of course, fully functional (except that they are no longer steam tight!).
I have replaced the pulley on the crankshaft with a Meccano sprocket so that it can be motorised. The metal base is mounted onto a wooden plinth because the flywheel projects below it.
I have also sectioned a Mamod SE3, complete with boiler and it's fittings. (This can be seen here.)