I don't know how to open the motor block safely.
I've converted 2 2085D Mallets from the 3 -> 4 wires.
You should be fine just opening the black gearbox blocks rather than the silver and red buhler motor casing.
If I remember correctly, it's the 2 outer screw on each bogie then the bottom half, with all the wheels and motion should just come away as a unit. The parts should stay in place if you lift the loco off the bogies rather than the other way round.
The motor and the 2 idler gears can fall out at this stage, but so long as you don't lose them it's fine and obvious where they go.
If you haven't had the gearboxes apart at this before you may want to consider clearing out and replacing the old gearbox grease, which may date back to the 80s and check the idler gears are in good condition. (for the 4 Mallets I've owned I've replaced 3 gears, 1 gear had been destroyed by the worm, in a second, the gear had been replaced by a previous owner with a wrong gear, and a third gear was missing altogether)