Haven't done this work yet, but read a lot of the archive items regarding the brakes, so following is just general background I guess. I take from your description of LHD and dual circuit you are in NA, so US suppliers preferred? Also assuming you have assessed MC condition and require total replacement rather than a repair kit, which may be commonly available from the usual suppliers.
Here is the link to White Post John mentions above as source of rebuild. From the archives, this appears to be the easiest route for a refit, with good reports on service. Could search on White Post to check out.
http://www.whitepost.com/brake.htmlChecked with Ray at RD Enterprises and several archived posts. Appears a total replacement for the dual circuit MC in absolute stock form is NLA.
Although Bean lists them on page J11 of my catalog
Lotus P/N 050J-0676
with 0.875" bore,
casting number 64676717,
vertical flange,
vertical? reservoir,
RH port location,
rebuild kit 050J-2485,
reservoir kit 036J-9496
Would have to phone to see if they can still supply or recommend a substitute.
The replacements that will work (Ray has one in stock I believe, Bean can probably supply as well) have the MC fittings on the opposite side of the assembly, and may be AN rather than UNF. I believe they use the plunger clevis from your original as well, so don't throw anything out. They will therefore require some plumbing mods to fit. I understand it is a similar issue in the Europa world, and Banks supplies a similar substitute.
The other archive options mention Tilton and Wilwood as possible modifications, although it gets a bit confusing between set-ups with and without boosters and different MC bores, single vs dual systems, and MC fitment for LHD and RHD, vertical vs horizontal flange orientation, flat vs. V shaped reservoir, bore size for Plus 2 vs Elan vs. modified Elan to Plus 2 brakes, proportioning valves, etc.
HTH. Please let us know how you resolve. Just in process of installing new lines on a bare chassis and hoping all the moving bits will work. Might need replacement in a few months if they don't work.