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 Domain{{`Keys (in the same file), you will need to install Mail::Domain}}Keys 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::Domain`Keys which stops it from working with SA. Patches to fix this are available at http://issues.apache.org/SpamAssassin/show_bug.cgi?id=4623
The *SA headers X-Spam-Flag, X-Spam-Checker-Version, X-Spam-Level, and X-Spam-Status are only added to messages that have been flagged as spam. I have been unable to find a way to override this and the always_add_headers configure option is no longer supported. This may be a result of SA no longer rewriting messages. As I understand it messages are left unaltered unless they are flagged as spam in which case a new message is created and the original one is attached to it so that it can be more easily used for bayes training and the like.