I think some more pictures of the Lathe may help, The viking name hasn't inspired recognition, but quite a few lathes are painted different colours and have brand labels stuck on them but are fundamentally the same machine. Someone on here may recognise the overall lathe under a different brand and the manual may exist online.
Hope the ebay chuck fits, but if not take the 2 prong drive you just removed to an engineering shop and they will tell you exactly what the thread is, and while you are there get them to skim it and remove all the damage done in removing it