You need to realize that the version you have was written for the server versions at the time.
Over 15 years ago and during that time the UBB software was changed to accommodate the newer server platforms.
So now you need to find a platform from 15 years ago that it will run on.
Only then can you contemplate upgrading.
And it will take several steps.
Also I do not think even if you get it to run , it will not run on a current hosting plan.
Unless you upgrade it to 7.x. probably UBB 7.7.4
At least unless the host has some very very old server software they use.
Which I doubt.
A lot of people got bitten by the host that updated the server to MySQLi and php7.
Prior to UBB 7.7.4

Had this been done over the last several years it would not be a issue today.

This is a big project regardless of how big or small the forum is if you want to preserve postings.