My opinion on this: Preservation of the old content must have 100% priority. This site is not a simple chat type of thing where information looses its value over time. Instead, it has grown to a huge knowledge base around the Amiga technology. Whatever Amiga-related problem you encounter, you will probably find the solution here. So, migrating to a completely different CMS and starting from scratch really isn't an option.
AFAIK, lower 5.x PHP versions have a compatibilty switch that enables PHP4 compatibilty. I'll see if I can find it somewhere. If this is not possible for some kind of reason, I suggest going the way Chaoslord mentioned.
Chaoslord wrote:
There is a migration path from Xoops 2.0 to 3.0.
There is a migration path from Xoops 3.0 to 4.0.
There is a migration path from Xoops 4.0 to 5.0.
There is a migration path from Xoops 5.0 to 6.0.
Loosing classic Amiga browser support is still better than loosing the content. The information collected here over the years is invaluable.