The Hymeks (and the Warships) were at an early disadvantage for freight working because the Modernisation Plan's promise to get rid of unfitted freight working didn't actually happen; their light weight resulted in a shortage of braking ability which wouldn't have mattered if the wagons had had (vacuum) brakes as expected. This was especially significant for working unfitted coal trains down the Welsh valleys, and even the WR themselves had to admit that the 37s were better.
For passenger work it was the other way round - the 37s needed regearing to make them properly fit for passenger work, but the Hymeks would make a decent shot at King schedules.
The Westerns on the other hand were very good heavy freight locos - it had been part of the design requirement. They were heavy enough to be able to brake unfitted freights adequately in their own right, and of course acquired air brakes later. Having the three axles on each bogie mechanically interlinked made them much less vulnerable to wheelslip than the 47s. They even apparently had some capability for continuous controlled slip (possibly due to the crawl speed control function on their transmissions), which diesel-electrics didn't get until the 59s. The only thing they didn't have was multiple working, but they seemed to manage pretty well singly.
The possibility of fitting air brakes to the Warships was considered at the same time as the Westerns, but they concluded that there wasn't enough room to fit the necessary air reservoirs. This had been the more awkward part of the job on the Westerns, and they ended up with tanks stuck in all sorts of odd places (and getting in the way) to get enough capacity; with the Warships they reckoned they just couldn't do it. I would hazard a guess that with the Hymeks also being notoriously cramped inside they would have had the same sort of problem, though the compressor could probably have been fitted in place of the steam heat boiler if it came to that.
The possibility of fitting ETH to the Warships was also considered and thought to be a goer; D870 was held back for a couple of months while they modified it to accept the generators and other equipment, but then they suddenly changed their minds and none of the parts were actually fitted except the ETH jumpers. Maybe that same change of mind was why they did not also investigate fitting it to the Westerns. In this case I would hazard a guess that if it was relatively straightforward to do it to the Warships, then the Westerns would probably have been similar or a touch easier, and it might even have been possible on the Hymeks, though that really is a guess. Both Westerns and Hymeks had at least in theory capacity for the rating of the engines to be increased to partially compensate for the ETH loss.
The answer when North British went under in 1962 was that Swindon (and even to an extent Laira) got stuck in and worked on a combination of manufacture, and also making the parts from their own Warships fit. In a way similar to what Eastleigh did with Class 33 when BRCW likewise went out of the business at the end of their construction, which there isn't so much fuss about. Swindon was aided by the original German designs from the two manufacturers there actually being designed to be completely interchangeable, and indeed it was quite common for German V200 Warships to run with one engine/transmission from each, so they were off to a good start.
NBL had set up a subsidiary (called Voith Engineering, I think) to handle the manufacture of Voith transmissions, which stayed around after NBL itself had gone down the tubes and took over the supply of MAN engine spares as well. This continued manufacturing until 1970, so it wasn't until then that it became necessary to get parts from abroad.
The Warships were originally intended to have the same interchangeability as the V200s, but it never came off. NBL modified the underframe design slightly on their batch to move the transmission mounts a few inches, but Swindon kept to the original drawings and reckoned on dealing with fitting Voith transmissions to their batch if and when it came to that, and in the event it didn't (though it would not have been a huge task if they had decided to restore the interchangeability). The engines were still theoretically interchangeable and at one point Swindon decided to fit MAN engines to one of their own machines as an experiment, but then they cancelled it without actually doing anything.
There were also problems with interchangeability of more minor components, where nominally identical parts could turn out to be incompatible because of the suppliers not following the spec, so a supposedly straightforward swap could end up requiring considerable fettling to get the thing to fit.
I suppose the underlying problem must have been that the intended repairs and maintenance system for the Warships never really got properly going; it was overloaded from the start and it was a struggle just to try and keep on top of the day-to-day work, and I guess they thought that while the interchangeability might have been theoretically advantageous, trying to make use of it before the system was running as intended would just have been another problem they could do without.
