FDC IC (U1) - Either National Semiconductor PC8477 or Intel 82077AA FDC ICs can be used. These ICs come in several versions:
National Semiconductor PC8477BV-1 - is the later version of PC8477 FDC and it is the recommended IC
National Semiconductor PC8477AV-1 - earlier version of PC8477 (a couple of subtle differences are listed in PC8477B datasheet)
Intel N82077AA - original version of 82077AA
Intel N82077AA-1 - later version of 82077AA it includes tape support, but FM support is broken (Note: FM is not used by any IBM PC disk formats, and only required for some older disk formats)
Intel N82077AA-5 - Similar to Intel N82077AA-1 but lacks 1 Mbps transfer rate support, which is required for ED (extended density, 3.5" / 2.88 MB) disks.