Doing conversions to MapGen is a little bit fiddly but fairly easy to get the hang of when you see the steps. I'm happy to make a video or document it, but ...
I've tested a conversion of the Meadow terrain and found an issue with how MapGen calculates the final colour palette. Any palette option lower than the 112 colour option reduces quality. EG, my images use 96 colours but the 96 colour option in MapGen makes changes to the colours of objects. But if you set 112 colours, it's likely to mess up the background gradient even if the terrain only uses say 50 colours, because it fills the rest of the palette up with colours that don't exist in the map. I'd rather this was fixed before converting the terrains I've made, only because I spent a lot of time optimizing the colours so the foreground elements fit within 96 colours.
Ideally it would be best if MapGen didn't give any colour options and just indexed the unique colours used in the map, leaving the rest of the palette empty. Maybe some detection code to warn if it goes over 112 colours and a separate "Save As True Colour" option. I've reached out to Plutonic on Discord.