Love this! Thanks so much for doing this!!!
Source is available!
Pushamo's source is now available as a downloadable if you already own the base!
You'll find two versions of Pushamo here:
The first one is decently cleaned and commented. This is handy if you care about reading the code - however, it cannot be exported and it is a slightly older version.
The second one is compressed, so that each cart can fit within the compressed size. (In PICO-8, you can use the INFO command to see this!). If your intention is to run Pushamo on more special machines like handhelds, you'll want to use this one because it will have some minor changes and bug fixes.
Feel free to create your own modes! The gamemode system is highly customizable; you can create any functions to happen after just about any event. Look at the arcade.p8 to understand more how this is done.
If you do make your own builds, you are allowed to distribute them, as long as you link back to Pushamo as well.
You are able to make your own builds for your own purposes, but not to distribute them without any significant changes.