As far as I know there are no more new meters unfortunately.
So either replacing the whole meter, or just the innards if possible, is right now the only option. There are 50uA movement from Simpson, the Simpson Wide-Vue (1329A or 1329T) series or the True-Vue (TV4TUA050 or TV4DUA050) series that can be used. they are not cheap but very nice meters and I prefer the Taut Band movements as there are no pivots/jewels/hairsprings in those, the 1329T & TV4TUA050.
I am working on a solution with a new replacement meter but Covid-19 has unfortunately delayed it indefinitely so I don't know when I will have time to work on it again. You might have seen this thread here on the forum:
https://www.vintage-radio.net/forum/...d.php?t=162294