I would rather like to know a bit more of the characters and they're pasts but some thoughts (note all Kanohi here are canonical):
Magnetism: Mask of Rebound, although his powers could replicate it's effects with metal weapons, so Mask of Accuracy would be better, or a Mask of Elemental Energy could also work if he ends up using Magnetism a lot to control his weapons in flight. Javelins or Hatchets as those will return anyway by his mask/elemental powers.
Iron: Mask of Strength - a classic, and would allow him to do good solid hits with his sword.
Gravity: Mask of Levitation, Repulsion or Flight - Repulsion is immoral in G1 canon but could be interesting to see a toa with such a mask and why, also could protect him from bad flail attacks. Levitation could also work, as he could use his Gravity to pull himself through the air and use the mask to glide safely down.
Plasma: Mask of Possibilities or Telekinesis - Help protect his team mates from the side effects of his powers and collateral damage can manipulating the battlefield. As a weapon - maybe a spear and shield combo? Keep him safe away from the danger.
Lightning: Mask of Truth, Detection, Conjuring or Emulation - I don't, just interesting powers. Weapons could a hooked/spiked chain to latch onto targets and electrocute them, with a dagger to protect herself at short range.
Lists of Canonical Kanohi:
Edited by Toa Jaxus, Nov 13 2017 - 02:55 PM.