I did not find a direct replacement for the S1055M at Littelfuse. It appears they have ditched their 100v and 200v (Vdrm/Vrrm) and the lowest they go is 400v. So long as the other parametrics check out, the increase in Vdrm is gooder
The part is S4055M and is in-stock/available at mouser.com . Here is a link to the datasheet so you can compare parametrics (to the original datasheet):
https://www.littelfuse.com/~/media/electronics/datasheets/switching_thyristors/littelfuse_thyristor_sxx55x_datasheet.pdf.pdf
NTE was my other thought. However their cross-reference tool appears to be a program that one needs to download and then register. I need to tend to some other things but if you want me to look at NTE just holler and I'll do so.