Unfortunately, you will either need to find a compatible Dell mobo or upgrade to a new case (standard type) and therefore a new computer. Dell uses proprietary mobos and connectors (front panel, USB, etc) in their systems, making upgrades very difficult.
There are some Optiplex motherboards that have the same BTX layout and will connect to the connectors and PSU.
I would recommend you look at txcesssurplus.com to see if they have something you can use. Good luck!