Industrial Singer 451K125, gearbox jammed?

The gearbox on this machine was caked in rust (although the outside was free from rust). The gear wheels (big one with internal teeth couples with small one with external teeth that connects with the rotating hook) are still jammed, after I washed out the rust. I want to disassemble this geartrain. How do I do this please? Anything I need to watch out for (like sprung parts that go ping when released)? Thanks