I cannot comment on John Borelli iSDK. @John_Borelli use to be a regular contributor in the community, but I have not seen him reply here for a long while.
@Marion_Dorsett2 recently uploaded a modified version of the iSDK with Service Account Key / Personal Access Token support here: Legacy Key Sunsetting - #39 by Marion_Dorsett2
If you still want to use OAuth, then you will have to investigate John iSDK. The Keap PHP SDK, which has official OAuth Support, is completely different to the Legacy iSDK, so you would have to make the necessary code changes to support it.