Michiel,
I don't know if you have found a solution yet....but I bought replacement Lucas headlights for my cab 11 from Angloparts.
You would have to check the diameter, as I am not sure cab 13 is the same. But new round headlights are still made by several manufacturers for British historic cars, but also for Lada's. This will also give you the option to choose more modern lighting if you want to.....and they offer the LHD vs RHD option.
https://www.angloparts.com/nl/search?q=headlight
Alex