Make a copy of the scenario´s eug-file and store it somewhere safe. Then load the eug-file in wordpad. Exchange the abbreviation of a country (HAB, ENG etc.) with one You like to play, e.g. STE for the Golden Horde (a complete list is in text.csv, first quarter, just copy it and print it out). Save the modified eug-file. When loading the scenario, the minor is available. (Don´t worry about the country selection screen still showing some text about the original country. This has no effects on gameplay).
Hartmann