You could check N900 schematics (wiki page) and determine, where magnet is located, then, give it to some *real* service center, enforcing them to check it first. I said real, as most repair shop guys are lame, and would either only try flashing and propose motherboard replace for ~300$, or, in worst case, damage Your device even more. Remember, that N900 is computer, and any random idiot that threat it as regular mobile phone may bring big problems.

/Estel