Hey,
The mod itself only makes a small change, so I'm fairly sure it can be made to work with 2.1 by just extracting its contents, adjusting the JSON file and repacking the mod.
{
"name": "Promethium-Stacks",
"version": "0.1.2",
"title": "Promethium Stacks",
"author": "AVADII",
"factorio_version": "2.1",
"dependencies": ["base >= 2.1", "space-age >= 2.1"],
"description": "Promethium Chunk stack size is now 100!",
"space_travel_required": true
}