The 420 is a BTX form factor, so start your search with that. You should also consider upgrading the Power Supply Unit, especially to ensure the P1 connector reaches the socket on the new motherboard.
Another thing to consider is that the front panel of the 420 chassis is a proprietary design, which means that all of the front panel connectors are attached to one large coupler to fit on to the 420 motherboard only. You'll have to find a way to splice new connectors on to the wire ends, after you've removed/cut of the proprietary connector on the motherboard side.