FROM: https://infinite.tcgplayer.com/article/Modern-Golgari-Rock-on-a-Budget/7acade58-202a-4542-bdfb-3949d88aa9e0/ 

<<Mainboard

2 Abrupt Decay, replacing 2 Eliminate

3 Thoughtseize, replacing 2 Agonizing Remorse and 1 Unearth

1 Collective Brutality, replacing 1 Cast Down

2 Hexdrinker, replacing 2 Putrid Leech 

3 Overgrown Tomb, replacing 2 Woodland Cemetery and 1 Forest

Sideboard

3 Fulminator Mage, replacing 3 Rain of Tears

The upgrades are straightforward enough, and offer better answers to the threats of Modern. Abrupt Decay and Collective Brutality offer versatility in dealing with threats. Thoughtseize is the premier discard spell and is what makes Golgari Rock incredibly disruptive. Alternatively, you can opt for Duress, Despise, or even Specter's Shriek until you can acquire Thoughtseize if you want that one-drop disruption. I'm a big fan of Agonizing Remorse, and it's as close to Thoughtseize as we are going to get for under a dollar. The downside is it can be awkward to cast on turn two, especially if you want to deploy a threat. >>