Well, I just played this and personally I think it is really well done.
Quote: " Furthermore, it would require to stretch the tiles to fit the screen and that would look really bad."
If you want to quickly test and see what it would be like if you scaled everything up, changing your virtual resolution to a smaller resolution should make everything appear bigger. Some of the map might go off screen, but for testing it should work fine.
Quote: "I use Tier 1 so I don't know if there's any other way to manage the editbox. "
Unfortunately not, the AppGameKit editboxes have always had lots of issues. Not to mention they don't support things like highlighting, moving the cursor, copy, paste, etc.
Quote: "a long war against framerate issues."
Just an idea, maybe after the level is generated you can store the coordinates of blocking objects (like hedges) in an array. Use GetImage to create one big image of the static geometry, delete all of the sprites and just use the new big image for the environment.
Sean