You will get a stack of documents from the post office. Don't fill anything in it right now. Take your ID, the papers and go to the Central Mail Exchange in McCallum Road, close to the rear entrance of the Fort Railway Station. Take some cash with you.
You should usually get the letter within 2-3 days of your package arriving at customs but may be things are delayed due to COVID. You can try calling them but they might not be very helpful until you get the papers on your hands.
The process there is very smooth. You will be given a token number and your name will be called when the package arrives. Depending on staff availability, you may be called to a different location on the same floor.
The customs official will value the package and calculate the taxes. If its for personal use, they don't bother too much. Its usually less than 1,000. The pistons may have got held due to weight. If you bike is high capacity, you might have an explanation to do, so best thing I can suggest is to say its for your grandad's old bike you are restoring. You don't need to answer any questions without them asking you.
I've got down plenty of things from overseas - car parts, home security systems and even a massive chandelier. Never had any problem. There's absolutely no need to look for kaakkas or anyone of that sort. Maximum I paid for a car part was 500 as I recalled. They can get nasty if you bring in commercial quantities or things that you shouldn't import. For example, when I was in the CME one time, a man had imported 24, 10kg cans of whey protein - apparently for a child dying of cancer! Customs can smell a bluff a mile away. So that guy had a time. We enjoyed watching the drama.
Here's a tip: dress well when you go there - shirt, trousers and black shoes. White shirt, black trousers, black shoes and a black tie can achieve unbelievable things in any government office!