Terraria is kind of a 2d version of minecraft that was made in .NET. The source has since been decompiled and posted online. Would it be legal to make a C++/SDL version of the game using the source as a base. I wouldn't be using any of the content from it so that shouldn't be a worry. I would just be using the decompiled source code to make a new verion completely written by me.

I was wanting to do this because the developer has stated they are done developing for it and they refuse to release the source or make it work in wine/mono. I would like to make it work on all platforms I could if possible.

Yeah I'll go ask on their forum about whether that would cause me legal issues, but I don't think so as long as I made everything myself and used a non-related title. I would just like to figure out before I start so I don't get sued if I release a version of it.

yeah with SDL you could tweak it to work on Android, Linux, Mac, iPhone, DS, and more. And if it was released open source I'm sure the community already around terraria would edit it and make awesome mods and additions that the devs wouldnt

One thing you could do maybe is some sort of patch, although I am not sure if IPS patchers work with all types of file and on multiple platforms.

I know some games like DOOM can be ported fine to other platforms even if only the demo version is available online for computers, but I am not sure how it works for discontinued games. I can't really help much since I don't even know if Terraria is commercial or not.

this is probably not the best of ideas, seeing as it is a closed-source game that is being sold. you could, perhaps, make said version, thought, and, once it's finished, offer it to the developer himself. if all the work is already done for him, he's much more likely to release it for multiple platforms, thus fulfilling your goal.