Sadly I believe that is a proprietary motherboard. It doesn't appear to conform to the ATX standard in either screw hole position or power supply compatibility.
*Edit -- I might be wrong on the screw hole position, the Dell photos look like a match for ATX, but the power is proprietary. There are adapters out there that will let you run a standard power supply. moddiy.com carries most of them as well as ebay and amazon (but less reliability there)
You can pick up a new motherboard, but that also means a new Windows license.
I suggest selling the entire computer as is and buying yourself an i3-8100 or i5-8400 / Ryzen 5 based parts.
To confirm, open the computer and take a picture. See if you can get all the mounting screws in the photo. Occasionally there are regional differences between Dell computers.