Thank you for your reply.
I have to say, I'm not quite sure I understand why this is an issue as I've had other forums remove posts without a problem.
The original post is from 8 years ago and the only post on the account. The topic was pretty much a throw away question. Even if I wasn't the same person, I hardly doubt the original poster would care. Saying that, as the forum logs IPs, you will be able to see that I am at least in the same area as the original poster. I can also PM you the email address that would have been associated with the account and any other identifying details you might think of.
Regarding Google, once a post is edited it can be removed from search results using the search console (the cached page is updated). The Wayback machine also removes pages at request, though I hardly think a page being indexed there would be an issue as you would have to have a link to the page and not just search my name.
While I perhaps understand wishing to preserve the board, would it possible to at least change the username of my old account?