I saw that seal with the 14mm ID on ebay it must be a missprint all the others on the net are 20 mm as is mine. The pinion shaft is about 20.5mm. I have two spare motors and there all the same 20.5 mm.
The 'oil seal' originally fitted predated the introduction of modern lipped seals and (especially when old) they are far less effective than a modern equivalent...
These are readily available and would be the best choice as an upgrade to replace the original type...Ian