Problems with control valves are few and far between but certainly within the realm of possibility.
I do not suspect the pump because you do not complain of a lack of ground speed going forward. Your complaint seems to be when reversing the tractor only. The pump has no idea which way the tractor is heading. It just pumps oil or it does not pump oil. It's a black or white situation.
The drive motor does know about direction but the usual complaint is that the tractor will back up quite well but lacks power going forward. This is due to the tractor travelling forward 90 percent of the time or more, thus putting more wear and tear on certain parts that are rotating while travelling forward. For these reasons, I do not suspect the drive motor.
The spool in the control valve may be damaged. The only way to know for certain is to remove that valve, dismantle it and have it inspected by a knowledgeable person. These valves can be commonly found on e-Bay for very reasonable prices. You could also contact antiquesetc on MTF's CCI forum because Ken parts out tractors and often has these valves.
You already realize the genius of the Case tractor's uncomplicated design, so there is no reason to relegate your 442 to that of a parts tractor when the problem can be solved rather easily and inexpensively. The Kohler is probably the easiest and least costly engine to rebuild and that needs to be done whether there is a minor issue with the hydraulics or not.
Take a peek at the 442 recently restored by Doug Wartil over at the Case, Colt, Ingersoll forum. Your 442 could end up looking just like his and I'm sure that Doug would be pleased to give you any assistance you may need.
I realize that you may be slightly frustrated at the moment but don't allow yourself to be influenced by the negativity of a couple of people on this forum. Every forum has members like this. If you truly believe that the linkage is tight, then why not buy a used valve, install it and refill the system with clean oil? That might be the quickest way to put your mind at ease so you can get on with the restoration process.