Nephar Whether or not you need to purchase a Spine license is independent of whether the product is intended to make a profit. You'll need to purchase a Spine license to use the Spine editor. You'll also need a Spine license if you want to distribute applications containing the Spine Runtimes or if you want to modify the source code of a game that uses Spine runtimes (Spine Essential is sufficient). Please see the Spine runtime license agreement for more information:
https://esotericsoftware.com/spine-editor-license#spine-runtimes
Please read this carefully.
I'm not sure what kind of modifications you want to make, but you could try to find someone with a Spine license to work on the project with you rather than purchase the license yourself. You can create a new post with the "Networking" tag to look for collaborators: https://esotericsoftware.com/forum/t/Networking
