You could easily make a visual solution for this.
You just create a circular sprite with Your desired radius, which You will make invisible, and create and destroy when necessary. Let's call it RadiusDiscSprite.
+For Each Enemy
+ If Chase = 1
SUB+ Trigger Once
-> Create/Spawn Object (RadiusDiscSprite, At Center of EnemySprite)
SUB+ Always
-> Set RadiusDiscSprite.Position to Enemy.Position
+ For Each Enemy
+ Enemy Overlaps RadiusDiscSprite
+ Trigger Once
+ For Each Enemy
+ Whenever Your Event sets Chase back to Chase = 0
+ RadiusDiscSprite: Pick By 'Closest to Enemy' (Or Position = Enemy.Position)
+ Trigger Once
- > Destroy: RadiusDiscSprite
I'm sorry, I don't know why indenting in my post doesn't work?