I've now started attacking this with the excellent CSSEdit for Mac OS. I can override the CSS for the page "live" which makes editing so much easier. I can also click on any part of the page to find out what styles are being applied to that element or block.

I'll need some accurate screen grabs or live demo of the old BBS as a starting point. I think I missed links to that, mind tossing them in here?

A number of things, as mentioned, involve more changes than just the CSS. Some logic needs to be edited for some and others might require some adjustment to the way the markup is written (nested tables and the like).

I will probably try a version specifying only the body text as pixels and then every other size as ems. That will allow changing all the text on the page simply by altering the base measure in the body element.