Could IR purchase the C919?
Only if it is free of any US components, including the engines (the CFM LEAP). I read names like Thales, Honeywell, CFM (as I mentioned before), Rockwell Collins, etc. being involved.
The one thing I don't understand is how is Meehan Air able to fly A340s and yet IR cant get even 2nd hand planes
Don't you mean Mahan Air? Either way, Mahan got those A340s most likely through intermediaries.
Which Westwern parts are used on plane?
I don't know the real specs, but if I read this correctly, there are e.g. components from Thales and Honeywell, and the engines are apparently based on the CFM56 (a US-French joint venture). The Russians would have reengineer a lot of the aircraft to avoid problems by selling them to Iran, and even then there's no guarantee that their US based partners won't pull the plug on the project just to protect themselves.
Sometimes the only thing more dangerous than a question is an answer. - Ferengi Rule of Acquisition 208