Hi - Sorry to hear of your space key problem. There is a decent vid detailing how to take the N900 apart here: http://www.youtube.com/watch?v=aVEHm325-z8

If I recall correctly, you need one small cross head screwdriver and a T6 screwdriver. The keyboard itself is pretty easy to remove. Best of luck sourcing a replacement.

Cheers,

Ian :-)