So long as your motherboard is no larger than 226mm tall and 173mm wide, it will go straight into the M1 without making any changes. (
Necere's Post on [H])
Mini DTX is about 190 x 173mm , i.e. Mini ITX with 1 extra PCI slot below where the ITX one is
I believe you can go a bit wider if you move your SFX PSU to the front of the case into the ATX mount with an ATX to SFX adaptor plate.
Sadly, I cannot test this as my used M1 came without the ATX PSU mount and some of the other accessories.