I'm keeping this around for archival purposes, but it definitely performs worse in Scratch 3 than it did before. So it's probably useless for everyone :) Drag the HP slider to change the amount of HP. Or press Q to lower the HP and W to raise it. Drag the MAXHP slider to set the max HP. Or press A to lower the MAXHP and S to raise it Move the Health Bar with the arrow keys. This is just to prove that it can be moved without lag.
Switched back to stamps because of lag with high numbers of HP. :D Feel free to use! :D This is for my next game! @MetallicDragon included this in his game! Check it out here: http://scratch.mit.edu/projects/23060291/