How to use stamp tool (older version of Sumopaint)
For example, after creating an object, head to the clone stamp tool.
When you have stamp tool's settings showing at the top, I suggest setting the diameter to 100px, choose the reticle option and then click on the object you want to clone.
Now choose the pencil next to the reticle. Find a spot in your image where you want to clone the object, while having the pencil option on, just click and drag.
If you still have trouble, you can Sumo's official Discord channel https://discord.gg/uDqTg2S9