That's highly unlikely, you can't change IMEI AFAIK.
And MAC:adress can't be change permanently, AFAIK. It can be spoofed but that resets after reboot unless there's a script at boot time but that will be reset when reflashed.

Anyhow I find that you are way too suspicious, for multiple reasons it's highly unlikely that he replaced your motherboard.

1) IMEI changing is not trivial.
2) Changing MAC permanently, could be done but not flashing-survivable.
3) Not that many N900 spare parts left in stores/servicecenters, if any. Let alone a whole PCB. If there's any it would be more assumable that is a cameramodule. (And he replaced the camera. )
4) What does this repairman gain by replacing the PCB? I don't see any advantages for him.
5) Soldering/desoldering eMMC not feasible without some very highend machines.
6) Changing lock-code can't be done without reflashing, so if he wanted to clone you're device he had to gain access to your device first which can't when you have a lockcode. Unless you had bootmenu, recovery-console/backupmenu installed, this won't happen without reflash.