What you need isn't the flag itself, but the actual files that correspond to that flag...
In Windows, the "System" partition is the one which holds the bootloader (Windows Boot Manager) and "Boot" is the one which holds the actual OS files. (Basically the opposite of Linux terminology.)
Windows comes with the bootsect and bcdboot tools which can install the bootloader into a new disk, although you need to create an empty to-be-System partition beforehand.
For 'legacy' BIOS boot mode, the System partition must be 1) primary (not logical), and 2) entirely within the first 2 TiB of the disk. It should be at least ~100 MB in size and NTFS-formatted. There are no special options for creating this partition, so just do it through DiskMgmt.
Once you have the partition created (and formatted as NTFS):
- install the BIOS boot sector using
bootsect /nt60 S: /mbr,
- install the Windows Boot Manager using
bcdboot C:\Windows /s S: /f BIOS,
- un-assign the drive letter.
If it's impossible for you to create such a partition on the new SSD, in 'legacy' BIOS boot mode the actual Boot partition i.e. C:\ can also be used as the System partition and hold the bootloader, like it used to in Windows XP; the steps of installing the bootsector and BOOTMGR are the same.
(But keep in mind that doing so would complicate a later conversion to UEFI boot mode, which does require a separate partition. If you ever plan to move this disk to a newer UEFI-only computer, a clean "native UEFI" reinstall will be a better idea.)
In your case, I'd probably delete both "Recovery" partitions as a way to make space, as the new bootloader configuration won't know where to find them, and Windows will eventually create a new Recovery partition anyway.
