I outlined the process for bricks in https://itch.io/post/4229594
In your case it is a bit easier as you just want to edit an existing brick, so you can use that as a starting point.
I don't think custom contracts should claim to be big missions unless they provide a similar level of content than a regular bonus level. That said, live tiles apparently only work for small tiles according to their resource names. But I haven't tried forcing them to be big. Garden show might be a reason to look into that again.