Hello @Mrkido
I am sorry to see that you are facing this issue.
Nikola boss does not respawn Heatran this way anymore, only if you had failed to catch it then it would have respawned in its place. As a result, it so happened with you and I have also confirmed that even if it were to respawn there, it would still have the same IVs as the one released.
Please do note that Nikola boss is not there to give you a free reroll anymore and it is only to get a chance at catching a Legendary you failed to catch.
How you can actually reroll your Heatran:
After it is restored, you can do it using reroll tickets at the Psychic Marina NPC in the following places:
Vermillion City in Kanto
Olivine City in Johto
Mauville City in Hoenn
Matsuki Village in Sinnoh
Guild Island
Now onto how we can restore your Heatran:
According to our TOOLS, you seemingly have transferred servers, from Gold to Silver, after releasing the Heatran in Gold. As a result, it seems to be stuck on the Gold server and restoring it right now would restore it to your 13 hour Gold server character.
Since that happened, you will have to transfer back to Gold and let me know when you do, and I will restore it to your box.
Have a great day,
Fusionflair