Veeery rough. The OC's animations aren't all lined up properly, some of Sonic's sprites still remain, and some stuff isn't recolored in properly. Ofcourse the physics and all that stuff is perfect, but concidering there's maaaany engines for fans to use nowadays and anyone is able to use them that honestly doesn't get any points, especially not when there's many things that have barely been made more custom. The special stage screen froze the game, credits left the game stuck on a black screen. The music isn't great, but the level design is... kind of alright, it's very Sonic 1 -like, but just more... let's just say lame.
The other songs were made by the composer Galaxy Hunter, and I really can't comment there.
I also have another question for you if it's not too bothering, did you feel the sudden change of the song in the first zone and the song in the second zone?