It shouldn't be hard to implement it. Here's an idea:

decide how far you're gonna display enemies (for example 1000 quants). Put an image and for every enemy that is closer than 1000 quants to the player pan_create a dot (save the pointer in an enemy skill, so you can move the pan) and have a function that checks the enemy position, subtracts that position and scales by a factor to fit on your radar image on the screen. When you subtract, if the enemy is (for example) behind the player, you get negative X and the dot will move in the negative direction. The only trouble is that, you have to center this position on the center of your radar...
If you want it rotating with your player, you'll have to multiply the final screen position with cos and sin player pan.
By the way, no need to convert in any way the X/Y of the radar, because you're already using X and Y in world coordinates, I think.

And that's it no math at all.