Should I power up my Zen headbutt/bodyslam Snorlax

Lucky you, this dude will rule any gym. Keep it and power it up

In terms of pure damage output as a defender Hyper Beam is only slightly stronger than Body Slam. In some fights Body Slam might even perform a little bit better. Some people claim that it is also somewhat harder to dodge than Hyper Beam, so it's definitely a strong defender. That being said, in the end it's all up to you. Some people are more conservative with their stardust than others, for me however a Snorlax (almost any kind) is pretty high up there on the priority list to upgrade.

I also have headbutt body slam snorlax with +15 attack, +6 defense and +15 stamina.