Have you tried to install it using Centos 5.5 as your base? MailScanner and Mailwatch hasn't really changed that much, so you might be able to get it running just following the original guide. You can get the latest version of Mailwatch from http://switch.dl.sourceforge.net/sourceforge/mailwatch/
If you don't need a virtual setup and just a straight gateway, you could have a look at The SpamSnake. You can even try Baruwa on Centos. Baruwa, which is similar to Mailwatch, is much more updated and works extremely well. Take a look at it, the documents will walk you through the process.