You could try rejigging this script by smallg, https://forum.game-guru.com/thread/207801?page=6#msg2505295.
Basically instead of using the red crosshair image when mousing over a target, you could use a health bar image, then say when g_entityHealth < 90, use a different image, when < 80 use another image and so on. Going to try this out myself and get back to you
