Things often take longer than planned in FOSS, and elsewhere.
Without some other compelling appeal to the idea, it sounds like the bare mechanics of Sonic without really anything new to say. Aside from the prototype code, I think potential contributors will need you to have some more thoughts on what makes running around jumping on springs and gravity-powered wheels
fun.
With Sonic, there was a challenge in picking from multiple paths and working out the optimal mix of speed-to-completion, rings collected, and enemies killed to achieve the highest score. Then there were the layers of attitude and appealing visuals, plus the enjoyment of maneuvering the character through obstacles and devices (which is the one thing your post points to now).
What games inspire you and have
ideas worth stealing? Are there any themes you want to explore or should it be a "pure" experience of running and jumping? What does physics add to this experience?