hello here axel srh and wanted to ask if there is a way to extend the level design from left to right
Refer to this guide. You can find the Level Size Array in "_inc\LevelSizeLoad & BgScrollSpeed (JP1).asm". If you're using REV00, you want to edit the file without "(JP1)" in it. Also remember to edit the level's size in your level editor's settings for the level. Note that there is a hard limit for Sonic 1's level format, which is 64x8 chunks.