UPBGE has a texture setting called “LOD Bias” with allows customization of mipmap values:
https://github.com/UPBGE/upbge/wiki/0.0.6#material-texture-lod-bias
Any value < 0.0 like -1000 will result in crispy, pixel graphics.
Or perhaps you would be more comfortable with using a pixelate shader to produce pixel graphics as a post-process.
https://blenderartists.org/t/pixelate-shader-filter2d-bge/593719