Just in case anyone else has run into this issue:
The Lucas 45D distributor is sealed with an o-ring where it enters the engine. I contacted our hosts, who couldn't find a listing on the o-ring and told me that the cork seal would work (which it doesn't). A standard 2-021 o-ring seems to fit, but the cross-section is .070" and doesn't provide enough compression to seal properly. I ended up purchasing a pack of 50 Buna-N o-rings in the correct metric size (M2 x 23.5, .079" cross-section) to fit the 45D distributor.

If anyone needs an o-ring and would like to make a small donation, pm me. I have a few to spare.