This addon adds several creative tools that allow you to quickly replace blocks. Use the eyedrop to select a block, and then use one of the brush tools on your world. Use the 3x3x3 or 5x5x5 brush to paint blocks in a small area or a path, or use the paint fill to paint over a large connected area or structure.





https://twitter.com/G2Crafted/status/1164220186876162048
I think a button or crouching to use tools would work