@Anon: These lists might help:

Free,_cross-platform_game_engines

Free, cross-platform, real-time_3D_engines

https://github.com/bebraw/jswiki/wiki/Game-Engines (only the ones with open source licenses like GPL, BSD, MIT, zLib)...

H...Homeworld... is that you? <3 :)

I recommend you start by creating a file to track files, authors and licenses, for example like https://github.com/bobbens/naev/blob/master/gfx/ARTWORK_LICENSE does.

The sounds seem quite RPGMaker-ish, would anyone agree?

There are quite some 16x16 sets on opengameart that will be able to help you. For a start:

http://opengameart.org/content/48-animated-old-school-rpg-characters-16x16

http://opengameart.org/content/12-battle-backgrounds-240x110

A good approach to replacing game art might be to divide it into seperate tasks and then to ask (for example here on this forum) for help to find replacments and converting replacements into the appropriate format.

Anon: CC FAQ's Is_Creative_Commons_involved_in_digital_rights_management? points out the following part of by-sa 3.0 (4.a.):

You may not impose any effective technological measures on the Work that restrict the ability of a recipient of the Work from You to exercise the rights granted to that recipient under the terms of the License.

To make sure this is being followed, I'm afraid you have to read and understand the entire license text.

My guess is that in Unity3D you do not use the format you found on this website but convert files into another format by importing them. If this format can be distributed separately, you probably will have to give users access to the converted file format, as it is a derivate work of the original cc-by-sa work.

Probably not on topic: notice that modifying the uv texture of a model licensed under cc-by-sa also requires sharing the changed version, if the uv texture is also covered by cc-by-sa. Creating a uv texture independant from existing cc-by-sa uv textures for the model might allow you to use a non-cc-by-sa uv texture on a cc-by-sa model, but it also might be that a uv texture can be seen as a derivate work of the model it belongs to, which would require the self-made texture to be cc-by-sa as well.

IWIWALNWID (I wish I was a lawyer. No, wait, I don't!)