G’day fxxxrr,
The welding thing on the donor is a bush fix. Seen it a couple of times. Probably not a bad idea form a ‘get it going and keep it going’ perspective if the problem is play at the key or missing grub screw etc. but makes purists cringe (I’m cringing!

).
You’ve probably checked this already but play develops between the gears when the nut on the cross shaft works loose. Needs to be snuggled up to remove as much play as possible. If it doesn’t have a nyloc nut then I’d fit one. For some reason the tractor models were fitted a plain nut and keeps working loose. Can’t remember what the red machines have.
If the gears are actually worn which definitely does happen to the later alloy gears and to a lesser extent the steel ones I guess then perhaps rotating the cross shaft 180 degrees so that the other half of the gear is now in use might work. Only half the gear is used and on later tractor models they used a sector gear, not a full gear. It’s an idea I’ve thought about but never fully investigated. On Dad”s machine this would mean rewelding the steering arm back on 180 degrees opposite as well but is do-able I think? Maybe rotating the steering shaft 180 so the other half is in the neutral position may help too?
How’s the weather treating you?
Cheers,
P.S. just thinking....maybe the cross shaft gear has slipped on the shaft?
EDIT. Just looked at mine and I’d forgotten about the circlip that stops the gear sliding so probably disregard the P.S.