Is there an option of liquid cooling/heatsink replacement for mid 2011 Mac Mini?
I will say no.

I haven't seen anything on the market yet. But since Mac mini is aluminum, you can just cool its casing. But that of course wouldn't be efficient.

The reason no one made one is the same reason no one made a water-cooling for laptop. The design of Mac desktops are more like laptops rather than a Tower PC. There's minimal space for anything else.