I see two directions you might take this deck in. One is the standard RDW route, which involves a mana curve that ends at three, and with a focus on slightly slower but more powerful cards. The staples of this right now are Goblin Rabblemaster, Stoke the Flames, Hordeling Outburst, Outpost Siege stuff like that. However, this is unfortunately the more expensive option.

You can also take it in the Sligh direction, which is about one and very few two drops and very cheap burn and buff spells. Naturally this is a deck that relies on overrunning the opponent and winning very quickly, so efficiency is key. You've already got some of the best cards for that in like Foundry Street Denizen, Lightning Berserker, Monastery Swiftspear and Mardu Scout. I think Zurgo Bellstriker is also worth having a few of. And Frenzied Goblin. As for spells, there's Lightning Strike, Searing Blood, Wild Slash, Hammerhand, Dragon Mantle, Titan's Strength etc. I'd be cautious with Stoke the Flames, at least don't have four of it here. And for lands, something like 17 mountains is the right number.

Hope this might have been helpful, good luck and have fun

I would cut Collateral Damage and Twin Bolt for Hordeling Outburst and Dragon Fodder. Getting a t2 fodder and t3 outburst creates a huge Foundry Street Denizen

I don't know if you're making it too budget friendly, but at least add a Nykthos, Shrine to Nyx.

nykthos is horrible in red deck wins