If you meet the terms of their licenses, you can re-enable DCC and Razor2 by uncommenting the appropriate lines in etc/mail/spamassassin/v310.pre .
If you decide to enable DomainKeys (in the same file), you will need to install Mail::DomainKeys if you don't already have it. It's available via CPAN or directly from http://www.cpan.org/modules/by-module/Mail/Mail-DomainKeys-0.21.tar.gz . You should also be aware that the API changed in version 0.80 of Mail::DomainKeys which stops it from working with SA. Patches to fix this are available at http://issues.apache.org/SpamAssassin/show_bug.cgi?id=4623