They can be manipulated in the HUDDATA.ini - but not sure you could get the shape you require?
Can you try BoxType=CIRCLE
If not you would need to locate any image files being used here and replace them - but its possible that the game just draws them at run time as they are basic shapes.
[Display001]
DisplayType=DIRECTOR
ObjectType=TARGET_ENEMY
ConeSize=0
BoxSize=0
BoxType=SQUARE
BoxColor=0.0,0.0,0.0,0.0
ConeModelName=
[Display002]
DisplayType=DIRECTOR
ObjectType=TARGET_FRIENDLY
ConeSize=0
BoxSize=0
BoxType=SQUARE
BoxColor=0.0,0.0,0.0,0.0
ConeModelName=
[Display003]
DisplayType=DIRECTOR
ObjectType=NEXT_WAYPOINT
ConeSize=0
BoxSize=0
BoxType=TRIANGLE
BoxColor=0.0,0.0,0.0,0.0
ConeModelName=
[Display004]
DisplayType=RADAR
PosX=0.0
PosY=0.0
Width=0.0
Height=0.0
Alpha=0.0
InitTime=0.0
[Display005]
DisplayType=DIRECTOR
ObjectType=RADAR_TARGET
ConeSize=0
BoxSize=0
BoxType=DIAMOND
BoxColor=0.0,0.0,0.0,0.0
ConeModelName=