Wanting to build up a flagging website

Hi all, I am new to this forum and have come here in the hope of finding the resources I need to rebuild my flagging motorcycle forum website. It was once a very active forum and has over 3000 posts, but the guy who attracted a lot of the members there (from another forum) died on a motorcycle ride a couple of years ago and the members have slowly drifted off since then.

The original drawcard of the site was the facility to post photos of your motorbike, describe modifications you'd made to it and post performance stats, but that never really attracted as much attention as I'd expected. It did well for a while fielding questions from newbies wanting to buy a motorbike or learn to ride, so I added lots of "how-to" articles and embedded interesting videos from YouTube.

I did a full site redevelopment earlier this year, moving the site from PHPBBplus to Drupal and Drupal forums (full conversion). I also added a glossy news front end to make it more attractive and introduced motorbike reviews and feature stories (like a news magazine). But in the process I think I killed off the last of the forum activity as I've not had one external non-spammer post since then. I do delete all spam posts and I have now turned on moderation due to the volume of spam posts.

The site still gets about 400 visitors per week but a lot of these now are spambots and spammers trying to post on the forums. I am running 4Q surveys on the site and those genuine users who respond to these are showing 80% task completion rate, and high satisfaction rate, but my bounce rate is over 70%, especially from Google. I earn some revenue from paid advertising, but only just enough to cover costs. I have recently added code that will allow me to observe user behaviors in the site and see where people might be struggling with either the design or the functionality of the site.

Some other things I am thinking about include buying another motorcycle forum site that has active members, recruiting some people to write motorbike and accessory reviews (although I am an experienced writer, I struggle to find time to do much of this), looking for some quality link exchanges with other serious motorcycle websites (I have some of these already, but not enough), buying backlinks to boost my Google rankings on key phrases, or selling the site and cutting my losses (but the value is dropping).

While I wait to accumulate enough cred to post in the right forums here, I'd be interested to hear from anyone who has suggestions about what to do to revive this website and rebuild its audience.