Could you rebuild the entire site in WordPress? Use instant WordPress and build your site from scratch. I am in the process of doing this with one of my sites. It is a big job and requires many 301 redirects with HTACCESS but I think it is worth doing.
Just set up a plan of action for your redirects using MS Excel or something similar to keep on track. If you undertake a large task plan out as much as you can in advance to save you having to backtrack later.