I have a 28 year old Sears heat pump. Bad accumulator. Repair or replace heat pump?

It has been a great unit. It keeps a temperature differential during the summer of between 20 and 25 degreegres F. I understand this is good. Anyway, do I replace the accumulator (400$) the heat pump outside unit and inside coil (2500$) or the whole furnace and heat pump together (4000$). Current furnace is great--occasional repair but works good (1972 model)