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It should be a Bottle Engine, to go in a bottle!
I remember seeing a small oscillating engine jn the science museum london that would have fit through the neck of that bottle! Building one that is bigger inside that at least looks like it none of it would fit through the neck is the problem.What about self assembly parts of brass plate with embedded miniature magnets ? A carefully finish pair of plates can be attached with almost no obvious joint , so a large base could be made from smaller pieces. Similarly, just as large steam-engine flywheels are made in section a small one could be made to almost self assemble with magnetic sections.The oscillating cylinder could pivot on magnet etc.etc.
I still say find a glass blower, cut the neck off, build the engine then reattach the neck. Bill
Good enough glass weld and who's to know Bill
You might overcome the self-starting problem by using a ferrous slug in a brass flywheel and an external (to the bottle) rotating magnet to remotely revolve the flywheel.