If the coil itself hasn't burned out (easy to check with a multimeter), a bit of cleaning could be sufficient to fix it.
Indeed a lot of the mechanical parts like valves, pumps, etc. will be available online from various places, and I bet you'd be able to find the same thing on AliExpress cheaper than Amazon.
As for why there are so many variations, it could be a form of vendor lock-in (like laptop AC adapters, where the number of different output voltages is far greater than the tolerance itself), or different companies just chose different OEMs.
Indeed a lot of the mechanical parts like valves, pumps, etc. will be available online from various places, and I bet you'd be able to find the same thing on AliExpress cheaper than Amazon.
As for why there are so many variations, it could be a form of vendor lock-in (like laptop AC adapters, where the number of different output voltages is far greater than the tolerance itself), or different companies just chose different OEMs.