Erm....I have a Bis...and both headlights suffered from rusted-out reflectors [is that the main reason for needing new ones?].
I have purchased [from Czech] new LHD headlights, which were cheap enough [circa 16 euros a throw?}
I have taken my original, rusted, RHD headlights, and carefully removed the glass lenses........by cutting around the sealant at the edges...bit of a dig about, but possible, watch fingers if using a stanley knife?
Once the glasses were out, I did the same with the LHD glasses on the new reflectors....
Then I got hold of some U-pol Tigerseal sealant. This comes in a cartridge, cheap enough off eBay.
[Watch the stuff, however, it really does stick things together fast and almost permanently...wear some sort of protection on hands, or be prepared to be picking black stuff off fingers for a week or so?}
The a small amount of Tigerseal was put into the ledge on the new reflectors, simply to locate and hold the RHD glasses.
The the glasses are inserted into the ledge on the LHD reflectors.......and more Tigerseal squeezed in, and smoothed off, [to replicate the original sealant]
Set aside to dry out thoroughly.
As it happened, I also obtained off eBay a sound RHD headlamp unit for about 15 quids.......which is a standby.
When messing like this, avoid any hefty levering to extract the original glasses....sometimes small chips can be made in the edges...not the end of the world, as the sealant will cover them anyway, but glass dust and tiny slivers can really be a health nuisance.
Anyway, come the day, MoT man will be happy with the shiny lenses.
Posh pointy halogen bulbs can be had that have the same fittings as the original bulbs, if needs be.....or, new LHD units can be had cheaply enough with the 3 pronged fitment.