I definitely suggest using that or another script, this is pretty bad and will cause you a lot of grief and hours in the future.
But, if you are adamant, then yeah, the first problem is you're setting the HP equal to the MaxHP, then changing MaxHP, which will do nothing.
Then, you're setting the HP before the MaxHP, which means there's no maximum value to hold what you're trying to add to it. You'll need to set the MaxHP first, then the HP.