I'm not sure what you mean about "building and maintaining a website forum"? You do know, that's what THIS site is, right? Are you talking about getting the old database from Tiki Central and creating a new site? The owner of that data already stated he will not let anyone have it. If you are talking about "scraping" all the data from The Wayback Machine, that has been suggested but from my understanding it is too difficult or impossible.
As for the 500,000 members, that is grossly inflated due to the admin not removing the spammer accounts. I asked the admin of this forum about that and he says this forum routinely gets spammer accounts as well, but he deletes/blocks them to prevent the forums here from becoming spamfests. If he left those in, I imagine the member count here would be well over 10,000 strong. The Tiki community has scattered to a handful of platforms. Reddit, Facebook Groups, and 2 Tiki forums that I know of. Once they scattered to the winds, it's tough to bring them back together.
Mahalo for the post!