if the steering gearbox is still working, good gears, etc: might try this fix:
1. adjust gears properly
2. suck all the old oil out as there is no drain in the gearbox
3. then fill gear box with john deere corn head grease or case ih corn head grease.
corn head grease in not really grease like most of us think of grease, it it more like a heavy oil and is one of the best lubes around.
it will not run out of seals like the 90 weight gear oil.
might try it,
taking that steering gearbox off and re-sealing it is going to be a job and no telling what you will break: steering wheel, steering arms, etc: realize those parts have not been off in 50+ years.
have him try it, who knows the corn head gear box oil might work and leak very little.
could also add some gearbox oil stopleak in with it again, anything better than taking that old gear box off and resealing it. another option: fill steering gearbox with grease, many old bushhog and rake, gearboxes running today that have not had any oil in years, when they started leaking: owner just filled them with grease and when back to work.