I have been playing around with the GTI Matt data.zip file. I can get it to switch to 2D and keep the upward heading orientation but haven't go it to work with overview mode yet.
All of the changes are in navigatemap.ui. Below are the changes I made, mainly changing the map.mapmode from 1 to 0. the .map.follow_rotation 1 may not need to be there but I have it there and haven't tested it without it. The map.SETCARPOSITION command sets the x and y coordinate of the cars position on the screen.. it is done in percents... 50 is the middle of the screen... 75 is 75% down the screen...
I will keep working on it and try to figure out how get the overview mode to work. If you try it and find any bug, please post them here. I haven't done a lot of testing yet.