First I'd ask why they abandoned it, you don't want to waste your time on garbage. Second, go to the version control and get the earliest version and code alon

First I'd ask why they abandoned it, you don't want to waste your time on garbage. Second, go to the version control and get the earliest version and code along. Understand each commit, why and what they're doing what choices/trade-offs are being made, read the comments etc. Then when you can see a function/feature being implemented try to do it yourself first, then go back to the version control and see the differences. This way you'll eventually have your own replica of the projects, and you'll have total ownership and understand every line of code. You'll also be able to contribute to the new project now that you'll have a proper understanding of what's going on.