When you have checked and fixed everything else in the steering system, if threes still play in the box you can probably rebuild it if you have the services of a machine shop.
Take it out the car, strip the box down, and check if the worm has corroded. If that’s not too bad you can replace the steel balls, they are cheap, and obtainable from a bearing specialist.
Before fitting the balls, next, get new shaft bush bearings made, fitted and reamed to fit the shaft which must be a close fit. Assemble the whole unit, and select the shims for correct end float and cover clearance (as described in the WSM if you can get one).
Finally use grease in your steering box as oil will probably still leak out even with a new O-ring.
I rebuilt my box this way and it works fine.