How would I gainaccess to the naturalisation records of South Africa of 1900 to 1920?

If the records still exist, I think they would be held by the National Archives of South Africa.

If you are interested in hiring a researcher to access the records, the following link includes some researchers who do military research http://www.casus-belli.co.uk/index.asp?PageId=133 . One I have seen rated well for military research is Rhino Research (as far as I know nothing to do with actual rhinos).

If you want to contact the National Archives of South Africa yourself, the latest email I have seen mentioned as a possibility is info@dac .gov.za . Most of what I have seen on Forums is about obtaining military records. Sometimes people are lucky, but if they are able to obtain records, it generally takes a long time. That's why researchers are often used.

The National Archives of South Africa website is https://www.nationalarchives.gov.za/