Goblin Swarm

If you wanted to make this Modern legal, I'd do the following:

-3 Beetleback Chief - Great card, but is the only reason this deck isn't Modern legal.
-4 Fanatical Firebrand - it's a 1/1 Haste for 1, but for the burn, you have to sacrifice it. We may be able to find better damage alternatives
-4 Wily Goblin - You won't need extra mana to ramp into anything, and your mana base well covers the low cost of the cards in this deck
-3 Akki Coal Flinger - Bloodmark's got the first strike covered, this is just redundant

+1 Ben-Ben, Akki Hermit - Gives you a way to damage your opponent if you can't immediately swing wide
+4 Goblin Chieftains - Because lord abilities are always welcome in go-wide tribal decks
+4 Goblin King - Because more lord abilities are always welcome in go-wide tribal decks
+4 Goblin Fireslinger - 1/1 with a repetitive burn ability can take the place of your Firebrand
+1 Krenko, Mob Boss - Exponential token production

I'd also drop four mountains and replace them with four valakut, the molten pinnacle. This gives you a way to damage your opponent simply by playing lands. They're not Legendary either, so you can have a few on the battlefield at once. They start to hit hard in mono-red, and coupled with go-wide Goblins, you should be able to do quite well.

Just some thoughts, hope they help!