This is a good looking SMB1 hack. Your custom ground tiles are excellent. I also like the idea of using Yoshi as the main character. The one criticism I have is that the Yoshi sprite lifted from Yoshi's Cookie is too small for the Super and Firey Mario states. Given that you're showing such skill at drawing new tiles, anywhere approaching 24x16 pixel sprites of Yoshi shouldn't be too tough. You might want to look at how Yoshi is done in Mario 3Mix, the Mario World NES project, or even Mario is Missing.