What the hell are they doing with your bike for 7 months? And what's the police got to do with it anyways?
If you have already paid ₹4000 to the workshop and its pending for 7 months, I can't imagine what kind of a maintenance work could it be.
Having said that, whether it is scrap or not, whether spares are available or not, irrespective of it all , they have no right to charge you and then sleep over your bike for thaaat long. Either they do the job or return the bike. Send a written legal notice by registered post asking them to finish the job within a week and deliver the bike or else, return the bike in the same condition as it was given to them, with refund of the advance paid by you along with interest.