What is a suitable substitute APS for obsolete Glow Worm Spacesaver boiler?

I am in need of an Air Pressure Switch for my Glow Worm Spacesaver 20-30F boiler. The boiler is no longer supported and the original part 202050 (Glow Worm part 419623 or G.C. part 381799) appears to be unobtainable.
Any ideas what would be a suitable substitute?
A part number would be a great help.

Thanks