Dragon Warriors

Has anyone repurposed Sorcerer spells and let the Elementalist use them in their games?

Specifically Dragon Breath and Fire Storm – what changes would you make, level-wise and effect-wise?

Personally, I wouldn't.

While Dragonbreath and Firestorm certainly fit the Fire Elementalist idiom, "giving" those spells to Fire Elementalists simply makes them alternative Sorcerers.
(Do you then give Shadowbolt to Darkness Elementalists? Do you give Tangleroots and Fossilize to an Earth Elementalist? What does the Water Elementalist get? Or the Air Elementalist?)

I prefer to keep the Professions separate and different. But that's just me.*

* I also like the limited canon of spells available. Adding an endless list of additional spells risks turning DW into Another Game(tm)...

Whole encyclopedias of spells...

Quite...

One of the things the Players' Guide did right was make spells take a long time to learn (with the time doubled or tripled if the text or tutor from which the spell is learned is not terribly good). They also allow Sorcerers any number of spells, the same for Mystics (which gives that Profession a boost); but Warlocks and Elementalists have limits set on the number of spells they can know.

The danger when creating a new spell is to avoid making something too useful; the spell everyone will want. Every Profession has a number of "lesser used" spells, which take a bit of creativity to use in play... I wonder if that wasn't deliberate?