I'm currently building a G/R deck and I originally was going to put in 2 Thundermaw Hellkites in the sideboard however after looking at what I have in my tradelist I say that this was not really an option for me at this time. Is there any ideas as what I can swap it out for?

http://deckbox.org/sets/363790

If you're really shooting for an Aggro build, you'll need to work in some pieces on the lower end of the curve. If anything, you should probably top out at four. As an aggro build really doesn't want the game to stretch past turn 5 anyway - as such, you'd drop Thragtusk and Wolfir Silverheart.

So that raises the question, what do you slide in? Well, cheaper stuff of course. Lose Arbor Elf for Experiment One. Other one drops should be in here too - Rakdos Cackler, Vexing Devil, or Stromkirk Noble are all great options. Disciple of the Old Ways should be replaced by Mogg Flunkies and/or Flinthoof Boar (don't forget Lightning Mauler too). Strangleroot Geist probably should be sitting in the mainboard too.

To be honest, I've never seen the need for more than 2x Domri Rade in a deck. You'd also be well served to slide in some Searing Spears either to clear the way for your creatures, or to hit them for their remaining life. Rancor can also go a fairly good way in the event they're sitting on a lot of blockers.

So, short answer after all of that being said, if you're going for R/G aggro, you should never even get to 5-CMC for Thundermaw to begin with.