Can I use Fairy's Hand Mirror to retarget the effect of Polymerization?

No you could not, as it does not target a single monster, which is what Fairy's Hand Mirror needs to be activated.

Ah, crums. Any ideas of what I could use for this?

Hmm well I don't have the card in front of me but I think you can redirect equipment to your monsters when your opponent tries to play them.

i used to use it on monster reborn and premature burial giving them the weakest monster in there

Not exactly what I was asking. What I was asking was what could I use to retarget Polymerization's effect. Is there anything I could use in either the American or Japanese game?

Oh, and Kurama? Fairy's Hand Mirror won't affect Monster Reborn or revival cards because the monster isn't on the field.

The thing you have to understand is that Polymerization doesn't target or single out anything until the card resolves. When it resolves you actually send the appropriate cards to the graveyard and special summon the monster.

There isn't anything available that can force the opponent to choose which monster you would like them to fuse for.

So nothing in the American game. Anything in the Japanese I can use? I heard about a card called, um... what was it? Magic... something... Magic Transfer, I think it was.

The 'anything' I mentioned was universal for the game, there is nothing out that can give you the desired effect.

No Japanese cards either, huh? Crums. All right. I still think their might be something out there, if not, then I'll just have to adapt it a little...