http://www.civicsolar.com/product/magnum-energy-ms2012-20b-pure-sine-inverter-20-kw-inverterFollow the installation instructions in the owner's manual and the supplement to the manual linked at the bottom of this link
It is unlikely that the DC wiring to the battery bank that the WFCO converter used is able to support the draw of that inverter.
You also need to keep or replace the DC run from battery to DC fuse panel that the converter used before so the charger portion of the inverter has a path to the DC panel as well as to the battery bank.
Probably you will need to re-locate the Magnum close to your (expanded for a 2000w inverter) battery bank and run fat wires to the battery bank, then leave the existing DC run from battery to DC fuse panel as the path for now both the battery and the Magnum charger to use to supply the 12v systems.
Once the Magnum is properly located for the 12v DC, then you can do the 120v side.
