I'm glad to hear you enjoyed the game! I do not have any sort of developed systems reference document. If you're familiar with the way the Mork Borg open license operates, 7:7 can be adapted under the same rules. https://www.morkborg.com/license/
Basically, (I am not a lawyer) it needs to include both the Mork Borg legal text and 7:7 legal text (below) as well as whatever might be needed for FIST. Like Mork Borg (paraphrasing their site) the mechanics and game rules may be reused and referenced freely; art and text may not be reused or translated unless you have explicit permission. I don't currently have any sort of "Compatible with 7:7" logo, though I should probably make one.
- [Product name] is an independent production by [Author or Publisher] and is not affiliated with Ockult Örtmästare Games or Stockholm Kartell. It is published under the MÖRK BORG Third Party License.
- [Product name] is an independent production by [Author or Publisher] and is not affiliated with Duncan Hall. It is published under the 7:7-A Dying Card Game Third Party License.
Does this help? If you have any questions or just want to bounce ideas off someone let me know :) I'd be excited to see your adaptation!