1. There were certain situations in the past where people might have lost their citizenship for certain reasons. You can look up the new law at
http://www.cic.gc.ca/english/citizenship/rules-citizenship.asp and go through it and see if you think any of those might apply to your relative.
2. If they are living in S-Korea for the last 60 years, odds are they are S-Korean now and have not bothered keep their passport updated. I have heard that Social insurance numbers become inactive if they are not used for a certain number of years but in any case they did not create the SIN until 1964 so if your relative has not been in Canada since 1950, they probably never had one. If you think the person is in Canada, I wouldn't know who you would contact to find them. In this situation, maybe it would be worth contacting a private investigator.
3. There are many websites you can use to search for people. The most simple is to type the relatives name into google and see what comes up. There is also 411.ca which is the phonebook, that is if the person lives in Canada but from what I understood, you say they have moved to S-Korea as a child.