Imitation may well be the sincerest form of flattery, but not where plagiarism is concerned. It can get you into deep trouble.
As the recent media coverage about Melinda Trump 'ripping off' a speech by Michelle Obama revealed, not only did it make Trump look silly, it also made her appear untrustworthy.
As publicists for mostly NFPs, we write speeches all the time. We work out the audience first and then apply messages to what the audience wants and needs to hear. We usually interview the CEO in order to get original copy, and to make sure the speech is written as naturally as possible.
When we work with speech writers, the process is somewhat different, mainly because we are one step removed.
So how do we make sure there has been no plagiarism? The easiest way is to google main phrases. But there are also some great online checking sites. Turnitin.com was one of the first tools, but there are others incuding:
https://www.grammarly.com/plagiarism-checker
https://searchenginereports.net/plagiarism-checker
And if all else fails, ask a teenager. Plagiarism is drilled them into as an absolute no-no. They will know the sites.