Arduino Zero Bootloader with no External Crystal ???

Good afternoon!

I am currently working on a custom ATSAMD21G18 board. Unfortunately, no external crystal is present (cost and space concerns).

I have successfully loaded the Circuit Playground Express binary from ArduinoCore SAMD repository, and it is basically functional. However, some of the pins which I require are not listed in its variant.cpp file. Furthermore, I have no use right now for the Python interpreter which comes with this bootloader.

Can anyone recommend a simple means of finding or building a bootloader identical to the standard Zero one, but crystalless? I have successfully opened the SAMD code in AS7 but am currently unable to build.

Furthermore, because the end goal of this project is production and sale, I will eventually want to release a third-party core with its own name and USB VID/PID.