Nice bike thats the way a kx500 should look
88 and 96 will be very similar frame/ only slight differences
The motor would bolt straight in but the 2 front motor mounting bolts will be a little big for the 88 motor 10mm vs 8mm[not a big issue}
They are a difference between the 88/96 bike though
The main ones are Rear shock / linkages / swing-arm / wheels / front end
Motor are different/ 2valve vs 3 valve top end/ bottom end is similar/ crank gearbox, clutch parts are all interchangeable