Great fangame. The visuals are beautiful, the music nice and relaxed, and I love both the inclusion of SMS stages (Bridge Zone especially) and the neat little between-act map screen intermission. A very charming remake of the original Sonic the Hedgehog.
This is the kind of thing I'd expect SEGA to do if they were smart and inventive, but oh well. My only criticism thus far is a matter of friction, namely in Spring Yard Zone. You know those down-slanting corridors with the floating spike balls? When I try to jump over them, I encounter too much friction with the cieling and fall right on or dangerously close to the spike balls. Also, if you find yourself with some free time in the future, I'd do some recoding of Jungle Zone Act 2 and the boss in Act 3. For Act 2, don't make the screen scroll up and kill you if you happen to fall back down to a platform you know is there. It is challenging, but I recommend implementing a scripted auto-scrolling event instead of scrolling up based on the player's ascent. Either that or just adjust the camera normally based on the player's movement. Also, Jungle Zone Act 3's Robotnik encounter is too easy to abuse. This may have been the same in the original game, but it's an opportunity to try and apply a solution that makes the boss less prone to abuse.
Also, there is that odd password bug I encounter when I complete Spring Yard Zone Act 2 and enter the Special Stage ring. It won't progress, and pressing C brings up a password prompt. Good work, man, and good luck. The music is a big improvement over your last version(s).