The theme still probably works, but is rejected by newer versions because of security enhancements, it could probably be forced into being imported for use. But even if you manage that, eventually you would need to update the theme anyways, it would only be a short term solution (as features won't work, just for visual).
I understand, I accept I will likely lose the theme and start again. I suppose I could find a solution,.
In terms of migrating though, is there anyone who can help me keep all the old users and ideally the posts from the first site, MyBB 1.6 php 5 over to the new installation of 1.8 on higher php 7
Well...
After reading this thread...
In all honesty, if I was in that situation, I would do a brand new install of the latest iteration of MyBB.
You did state that the forum did "die" due to lack of admin control and it was taken over by bots and unauthorized usage by whoever broke into the website.
On top of all of that... you don't know how much information those unauthorized users obtained from the user list and quite possibly... also obtained the user's passwords.
There are way too many red flags to upgrade because there may be some residual files that may still allow that unauthorized usage and you wouldn't be aware of it until it's too late.
It's too risky, in my opinion, to upgrade.
My best suggestion is to start fresh... brand new... then, you can modify the themes and add-ons to your liking.
You can even install a few anti-bot/spam add-ons that will minimize your dealings with bots and spammers.
Then, you can open up the website, but alert people that they will have to make new accounts using their user IDs and passwords.
Just my measly 2 cents.