Perhaps it would be easier to insert the maps of Magna Carta as a provincial campaign in the RTR mod.
At first sight you should copy the whole folder of Magna Carta and edit there descr_strat and descr_mercenaries with the units of RTR.
You must also include the Magna_Carta_regions_and_settlements_bla_bla in the text folder.
The major problem will be the rebels. It would be necessary to include the new rebels in the corresponding files (descr_rebel_factions, or something like that), checking that all the rebel factions in Magna Carta are collected there.
Perhaps I didn't explain it very clearly, but I have not the game here. Sorry!
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