This will not be easy, but also not as bad as it looks.
You will have to remove the staking for the gear side truck side only, with a small grinder(Dremel, etc.) leaving enough material to cement it back after the repair(12 hour epoxy).
If you pull the two wheels, and maybe not even all the way, you will be able to replace the two worn nylon idler gears.
The gear studs should be fine, the nylon gear design was implemented in 1970 to allow gear wear without the brass-on-aluminum/steel and steel-on-aluminum/steel postwar(1955-1969) wear to the axles/studs that rendered those motors junk once worn out.
The rest of the gears(the worm wheel and drive gear) look good, and I wouldn't dis-assemble the truck any further as that would entail doing some irreversible damage to the original staking, and it would never be quite right again.
Clean all old grease & debris from the entire assembly. After pressing the wheels back on & gauging them, cement the truck side frame back on, you can re-stake if possible.
Lube it right. Use synthetic motor oil on the brass/Oilite axle bushings working it in well, and Lucas Red 'N' Tacky #2 on all side gears(and those two little idler studs), the worm(armature gear) and worm wheel, as well as the lower armature bearing below the worm gear.
The condition of the traction tires, rollers, and worm wheel point to a low-hours diesel loco, it was just run dry damaging the idler gears.
