Why do you want to preserve that old pc case?
Does it have some sentimental value or something?
Anyway - Dell OptiPlex 3020 comes in multiple form factors.
There is SFF and MT form factor.
Which one of those is yours?
mITX form factor board would fit into both cases (SFF and MT).
In MT case you can also put mATX motherboard.
Note Dell motherboards use non-standard PSU connections.
If you replace board with standard motherboard, you will not be able to connect Dell PSU to it.
You'll need a new PSU also. Then again standard ATX PSU will not fit into SFF case.
So you'll be screwed either way.
Just be prepared to get a new pc case and new PSU with your new motherboard.
That will be way easier than reusing old Dell OptiPlex 3020 case.