It can't run in a window and tabbing out causes major problems so you can't add it to OBS

I don't know if MDK uses nGlide 1.05 or newer but, in that case, or if you install/upgrade it, hitting ALT+Enter should be enough to switch to windowed mode.

For running MDK 2 in windowed mode, I think you have to use Borderless Gaming.

To clarify, if you install the latest version of nGlide then delete any glide .dll files from the game installation directory, you should be able to play in windowed mode.

Unfortunately, this only applies to the 3DFX version, but the lower resolution textures shouldn’t matter too much for streaming. The D3D and software versions would require more finagling, as there are bugs with this game and the latest version of dgVoodoo.