Sulston Hills do the solder repair, but I opted for a new (used) motor. After stripping down a motor, I can understand what they were driving at, but I suspect the repair will be a bit hit and miss, and might not be that long lasting. The motors are widely available, but Autolinkuk seems to be the only place that’s acknowledging different motors exist. When hooking the motor up to the linkage, its fairly critical to have it correctly fitted to the splines, so the wipers park correctly.
What was wrong with all the other motors you brought? I can only think the new motor you brought was for a late Mk1 or Mk2.