To understand why you cannot find the ROM, consider the hardware limitations. The Neo Geo MVS ran on a 16-bit Motorola 68000 CPU at 12 MHz. The Nintendo DS ran on two ARM processors (ARM9 at 67 MHz and ARM7 at 33 MHz).
Because no Neo Geo ROM exists, players looking to experience Metal Slug 7 must use the following official platforms or their respective ROMs/images: Key Features Nintendo DS Original release; single-player only; unique map screen. Metal Slug XX PSP, PS4, Xbox 360, Steam metal slug 7 neo geo rom