The ST7920 display controller is supported by u8g2: Use constructors starting with U8G2_ST7920_128X64 or U8G2_ST7920_192X32.
However the U8X8 interface for the ST7920 is not avilable (it is actually disabled for the API), because the memory architecture of the display differs from most other displays. U8g2 will still use u8x8 internally, but you can not draw text via the U8X8 interface.
There are several tricks to reduce memory consumption. Most should be named in the U8g2 FAQ. This should include: Use reduced fonts (ending in"r"), disable I2C (due to a problem in the Arduino libs, I2C is included although if not used by u8g2), use one page buffer mode.
Oliver