Rowdy usually sells, T51 and T60 (non-Atom Cat) parts. She will also fully restock T60 parts with the Atom Cats paint job periodically (I think every 48 hours). ON OCCASION, she will also have T45 parts and/or X01 parts in her inventory. Keep checking back with her.
{Edit} From my experience not all power armor spawns as level dependent. As an example: No matter what level I am when I first go to Concord, the power armor there is always T45. Additionally, the "leveled" power armor seems also to take the Difficulty game setting into account. That is to say, a power armor that is X01 Mk. III at level 35 on survival difficulty will be T51 at level 35 on easy difficulty. Rowdy may function similarly (I haven't tested it).
So try this: Go to your home base. Set the game difficulty to Easy. Sleep twice for 24 hours. Save game and then travel to the Atom Cats garage and check Rowdy. If she doesn't have the T45 helmet you want, load your save and try again. It may work. Just remember to set you game difficulty back to wherever you had it before you started meta-gaming.