Did you check the Flottilla.cfg file to make sure the XXI is available. Also i didnt notice you saying you had added the years 1939 etc to the XXI in basic.cfg or changed the xxi's year of availability.
I did a mod like this a while back myself, i only had to work with flotilla.cfg and basic cfg.
Btw this is a modding question, it should be in the sh3 modding forum
