What fuse can I use to replace a Hitachi FN00373
Turns out that "51MS" is an entire family of fuses, all with the same basic characteristics but different current and voltage ratings. Assuming you can't find another "51MS-050L" from another manufacturer to replace it, you should be able to get away with any leadless, 5.2mm×20mm, medium slow-blow fuse rated for 5A and at least 125V.

Source 1

Source 2

Source 3