You can't edit the actual shape of the stadiums, just the courts (with
this program).
You can edit the texture however you want tho. Use
BigGUIto open the .viv files and extract the .fsh files of the thing that you would like to change; then use
FshX to open the .fsh files and extract/import the textures.
There is a way to remove the crowd (not to edit it tho), but it ain't easy; and if you want to do it for performance issues, it doesn't do any difference for the performance.