Have a look at this : https://geysermc.org/

I used it a couple of years ago and it worked fine.

> So it's based on the original code? But it's also said to be open source. I'm confused.

The contents of the open source repository are a set of patches to apply to the decompiled code, plus some fully new classes to add an event system, or interfaces (where patches hook into the event system and provide the implementations of the interfaces)

Also most people use Paper (which is a Spigot fork that recently diverged and became a hard fork)