Since someone asked, here is size and part number information for the crankshaft rubber front seal for late production MGA, all MGB and some MG Midgets.

For MGA the BMC part number is 2A939
Size is: 1-9/16" ID x 2-15/16" OD x 5/16" Thick.

Alternate part numbers:

BMC	2A939; 12G791; 13H4061 13H6516 13H6517; 88G561; CNA473; OS5910; S5910; SC-1189
GACO	INC 45894
Moss Motors	120-000
Victoria British	5-834

Someone claims that a 3/8 inch wide seal will fit, but I haven't pulled a timing cover to verify it yet. That width can be this part:
National 1873 -- 1.562 x 2.326 x .374 with Nitrile lip