The original PIC source code is now available for study at https://fossil-scm.org/pic/.

I received this code directly from the original PIC author, Brian Kernighan, together with permission to post it. But its license is unclear. You should probably not reuse any of the code in other projects.

But its license is unclear. You should probably not reuse any of the code in other projects.

But studying the code before writing one's own (while making sure to avoid copying solutions verbatim) is supposed to be okay, right?

But studying the code before writing one's own

That's a gray area. Much safer to have some one else study the code and create a specifications document. Then design and implement new code from the document.