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(+2)

OK this is well cool. I'm not particularly technical but is there a way to turn an existing city into something that works, i.e. how to convert a graphics file into JSON?

There is no way to convert graphics into this JSON format. You can try to recreate a city you need in MFCG i.e. reroll a map until it's close enough to your idea and then tweak it using the Warp tool.