Brassuer trains (thetraindoctor) has the exact part number you are looking for that is the knuckle and rivet set for the k-5001 truck. I know this because I was going to purchase the k-5001 truck he has from him and saw the number you are looking for in stock but found what I needed elsewhere. I think it was only a few dollars but shipping may get you. This was just 2 days ago.
Brad