Ive got a similar question. My fiance has a Magna MTB that the seat was stolen from in similar fashion while it sat outside last year at her old apt.

I have tried buying a couple posts at a time (they cost me about $.25 after my volunteer discount) from the co-op and taking them home to try them, but so far, the closest ive been able to find is a few milimeters too small and slides down to the bottom of the tube. Im going to take her bike over there and try every post in the shop on saturday, but if that doesn't work, any thoughts? I dont even know where Magnas were/are sold, and she's had it for at least 8 years.