Nerfing a hydra to be cr7

I want to add an optional hydra fight to a module im running (specifically "Frontiers of Ebberon: Quickstone" 's adventure heart of stone), with 5 people. But the module is lvls 1-5 and lvl 4 has the best opertunity for me to add this but to work in level 4 id have to nerf the hydra, i was thinking because its a desert setting instead of hydra's usual swamp habitats i make the hydra regenerate heads after 2 rounds and reducing the hp to 153 from 172, giving it less health and cutting the healing it can get in half, also allowing the party to use the fire tech to stop the heads from regrowing which i will give them clues or just tell them about if they do a good check, but don't know if all that will be enough.