IMHO You have to take receipt (original) from the olx seller to make sure engine do not belong to some "stolen" vehicle and also have a signed receipt of this deal to with NIC photo copy attached to have a proof that you bought it from whom.
In your case you bought engine which was sold by me to @taha66 who later sold it to zunair in ISL from whom you bought now ? right ? I gave the orignal reciept to Taha when sold it after using it for a while in my bike becuase I upgraded to 250cc variant.
As far as I rememebr taha handed the reciept to zunair as well, I epxect that reciept would be a wet piece of paper if not presered properly becuase it was a normal sale reciept of the shop where the engine was bought like 2 or more years ago...
hope that helps.