Which Pebble are you bringing back? I ask because Pebble 2 was the sweet spot for me with a heart rate monitor but the heart rate monitor is not in this repo.
The HRM aspect is mostly a small binary blob that ought to be fairly straightforward to re-integrate. The far larger issue is the lack of a Bluetooth stack.
> This is the latest version of the internal repository from Pebble Technology providing the software to run on Pebble watches. Proprietary source code has been removed from this repository and it will not compile as-is. This is for information only.