Best Bet is to download a Linux Live CD, Load Up Gparted, You should be able to Determine your WIndows install by Name/Size/Etc....
When your done with that Download Grub2Win (On C:), And Ext2Fsd, Ext2Fsd is a windows program that lets you read/write to the EXT 4 partition, But also lists the Partitions..
Grub2Win, Load it up (Your Win 10 Boot will already be there) add your X86 Installed partition save it (Green screen when all applied means it worked), Then search here for the proper Grub entries for X86. Load "grub.cfg" find your X86 Entry and copy past the Info from here there. save, reboot...
Nice thing about doing it this way, Is your NOT anywhere near your Windows MBR, So When you reboot you'll Have "Windows" and "Grub2Win" Clicking Grub2Win will give you WIndows and X86 Install...
Personally what I'd do, Is Download a Linux DIstro, Burn it to a USB (Or Use DriveDroid on the PlayStore the have a Mint Image in The DL of the APP), Boot into it, and Install on the USB, Then Install GRUB Customize, And do the same as Grub2WIn...
I Have BOTH, So I have a Grub2 Boot That has Windows,Linux, X86, and Grub2Win, Which has The Same, As it's ALOT easier fixing Grub2 (For me)...
All This information, AFTER I've done NUMEROUS Format C: back in the day, Unfortunately made me REAL GOOD with Grub(s)
Hope it helps.
Mike