I tried to follow the example… but it didn’t work for me… I tried to look for other articles in the forum like these that were practically very similar to my problem
It seems that there is no clean upgrade-safe solution. (I don’t want to change the core code of the Lead module, I did it to test it and it works but it should be lost with updates)
I think the best solution to customize the assignment notifications is through WorkFlow, after disabling the default notifications from the system. B-)