Just to add some more info. Shimano shells are 31.0mm where the cups press on. Phil are 33.0mm. I just double checked. If Sunrace are 31.0 as you say, it could theoretically work.
That leaves only the issue of whether the width will match up closely enough. (and the spindle diameter, and the cotter pins... )
Unfortunately, Shimano don't AFAIK make a UN55 with Italian threads. IRD maybe?? It's also worth mentioning that the fixed side cup of a UN55 doesn't come off. I think Shimano used to make it in Italian, so perhaps you could find two of them used, and pilfer the removable cups. (since side cups are RH thread on Italian.)
Correction: Shimano still makes the UN55 with Italian threads. Probably fairly rare though.
Last EDIT: looks like that Sunrace is made like a Shimano, and the fixed side is fixed. Therefore the idea is a non starter. Sorry.
Last edited by Salamandrine; 03-01-20 at 11:46 AM.