OK. small update, to report that probably the first 512kb of the image is all that is needed for the game (even if mirroring it twice up to 1mb is harmless)
the unpatched 2mb rom consists indeed of 4 copies of the first 512kb chunk (crc32 5e4266a7), while the patched 2mb rom consists of a first modified 512kb chunk (crc32 10b63778) followed by 3 copies of the not-modified 512kb chunk, meaning that the patch only modifies the first block and that the game never requires the other blocks to be modified (maybe it never access them at all)
not sure if this additional understanding can be really considered as a step towards emulation of the "mapper" used by Yong Yong for this game, but it explains at least why patching the 256kb file results in a broken file