I've planted a variety of crops with my 71 units and they use the same plates as your 1240. For cantaloupe, watermelon, and cukes I've used the H1262B plate with the Y3313B filler. I've got a B27436 pumpkin plate and although it does work the cells are quite large and it drops a lot of doubles and triples with the seed I've used. This plate is used with the Y2630B filler. (Note that with these specialty plates the filler isn't optional - the correct one MUST be used.) I've never planted sunflowers but Lincoln Ag Products does carry a variety of sunflower plates and the required fillers. You'd need to measure your seed to determine which one is most appropriate. Also note that there isn't any hard-and-fast rule about what plate works for which seeds - if the seed fits nicely into the cells it will work, regardless of what the plate is advertised for.
I wasn't aware that Deere marketed the Planet Jr. seeder. These seeders used to be used in the area I grew up for planting vegetable crops. Surprisingly, this same unit is actually still being produced by the Cole Planter Company.