Am I able to use rolled hearts when attacking a monster in Tokyo?
You heal because movement into Tokyo only happens after the dice have finished resolving.



There are five phases to a turn.


Roll Dice
Resolve Dice
Enter Tokyo
Buy Power Cards
End of Turn


Monsters yield Tokyo when resolving dice in the second phase.


  If you are outside of Tokyo and you roll {Claw}, all  Monsters  who  are  in  Tokyo  lose {Heart} (Tokyo City and Tokyo Bay). These Monsters can then decide to Yield and leave Tokyo or stay. Monsters who Yield Tokyo still lose {Heart}.


But the active player only enters in the third phase.


  If no one is in Tokyo, you must enter and place your Monster in Tokyo City.




Even if attacking somehow forced you to enter Tokyo, you could still Heal by applying the effect of the hearts before applying the effect of the claws.


  You can resolve your dice in any order, but you must resolve all of them.