The only 3D Sonic engines I’m aware of are Sonic World and SonicGDK.
Sonic World is made in Blitz3D, based off of an old, old, old codebase that’s probably nearing ten years old at this point. It definitely has its fans, but, like… man, I dunno. To me, it’s kind of bloated and weird. Like, at this point it has something like 31 playable characters? And the build at SAGE a few months ago was… enjoyable, I guess, but also really awkward and janky. Like, to watch Ozcrashsonic play it, he’s accustomed to the controls and flies around levels like a ninja, but for a newbie like me it was kind of awkward to come to grips with.
It’s worth mentioning that there are, or at least were, a lot of other offshoots of the Blitz3D Sonic engine, but I think Sonic World has taken its place as the defacto standard that everybody uses now.
The only other one I’m aware of is Sonic GDK, which is based off of the Unreal Engine 3 UDK. GDK has what you’re looking for: all of the boost Sonic fixins (boost, quick step, stomp) on top of… well, a lot of stuff. Sonic Colors wisps, elemental shields, so on and so forth.
Here is a slightly outdated look at something that was made in Sonic GDK that tries to replicate the Sonic Generations look and feel as closely as possible. There’s a newer version of that level that’s out now that both looks a little better and plays a little better.
This is what base-level Sonic GDK looks like, though this video is also a bit outdated.
I don’t think Sonic World is open source, but Sonic GDK most definitely is. I’ve had dreams of learning UDK and spending a significant amount of time adding a bunch more stuff to Sonic GDK (rails, tricks, porting over every enemy I can find a model for) but that will probably never happen for a lot of reasons, some of which are out of my control.