My Two Choices

[Locke]

I'd say if you can't contact him whatsoever and there is no 'ReadMe'/whatever -file with the original mesh stating otherwise; Giving Credit to the original designer should be enough.
That is of course, only my opinion.

Only problem there is that we recently had a guy pilfering meshes from BC and modifying them, then claiming the work as his own.  Even when he finally did contact the original authors for permissions and didn't get them, he still didn't stop.  A sticky subject.

[FarShot]

It's not enough unless the author specifically states that his design may be used without contacting him first.  Keep working on it Locke, and send periodic status updates to his email.  No one wants their designs remade terribly, so showing some good work will help.  In the end, if you never reach him, just don't distribute the ship.  If you can't reach him by email, also try finding him on forums he may frequent, like Sci Fi Meshes for example.