Maybe take the piece back to TRU and explain the issue, sometimes when Toy Shops do returns for faulty goods they do not send them back to the supplier instead they retain the broken toys for part exchanges and or re-sale later as is. You may be able to convince them to swap the Spike with a faulty Magnus figure. Though truth be told with so few Magnuses going to each store it's unlikely a faulty Magnus will be available.
Alternatively because of the direct import arrangement TRU are currently employing to sell Takara MPs they may be able to engage Takara directly for a part replacement on your behalf.
Lastly if you ask nicely on here someone may be happy to give / sell theirs to you, some of the collectors here are not into accessories and will often sell them off (that's how I got my HA Mudflap)
All you can do is ask mate.