Hi,
Simple questions, is there a mod that can make the game get the multiplayer scaling on solo?
If not, would it be hard or even possible to make?
It can be done, I don’t think one exists yet. That said, it won’t work like d2 where you type a command and tweak difficulty on the fly, at least I don’t think you can do that with Lua.
There is a global difficulty modifier that affects all monsters, you’ll have to look for it and eyeball it to figure out what you need to do.
There is this:
By @adoomgod but I am unsure if it affects anything while solo. If not maybe he could add something to it?
I have tried doing it alone, but honestly I’m completely lost. 
I will ask @adoomgod , if he can’t help I’ll just wait for the next big update and try more seriously.
It does not do that. It’s a mod that makes multiplayer harder. Not something that lets you scale to more players while in single player.