Many years either side of your 1978. They changed the lower main seal cap in the mid 1980s moving the upper driveshaft seal from the driveshaft to the powerhead, but that's about the only major difference. You can swap most periphery stuff as needed to use an earlier one (recoil start-in-gear lockout is different, cylinder head is different (use your 1978), exhaust cover is different (lack outlet for pisser and mount for powerpack)). Later ones up to about 1983 or 84 should bolt right up. I think later ones would work, too, but you'd have to either remove the o-ring from the driveshaft or the seal from the lower cap of the powerhead.
