Will a 1986 kdx80 cylinder fit into 1987 Kdx80 engine?

While both are KDX80s, there's a possibility of incompatibility. While the overall design is similar, minor changes to cylinder porting, bore size (even slight differences), or the piston/cylinder interface could exist between the 1986 and 1987 models.

It's best to verify the exact part numbers of both cylinders to ensure compatibility. Checking a Kawasaki parts fiche (a diagram showing exploded views of the engine with part numbers) for both years would be the definitive way to answer this. Without those numbers, there's no guarantee they will be interchangeable.