Download Android Jelly Bean 4.2.2 stock firmware for Sanei N903 tablet from here or from here. Download another Sanei firmware from here.
Archive name: Sanei N903 Fw / N903(尚伊)V1.0改搜狗输入法的软件20140901
Firmware image file: [安培i-a9005qc_sanei_a23_v2.0_4.2.2_2014031802]_[8位800x480_ctp兼容_双Camera_DDR480_gsensor兼容_ddr528_odt].img
FW version: v2.0 / SANEI-N903-eng 4.2.2 JDQ39 20140901 test-keys
Archive file size: 259 MB
Model: Sanei N903 tablet
Platform: Allwinner A23
Build number: polaris_chiphd-eng 4.2.2 JDQ39 20140328 test-keys
Pixels: 800 x 480