The Marathon has a 65# weight limit, not 50#, but it is outgrown by height if the child's shoulders are above the top strap settings. At 43", that is a possibility.
If the seat is outgrown by height, you need to get a different seat, either a harnessed seat or a booster. If she has had no booster training, I would recommend a harnessed seat, preferably one that converts to a booster for later.
If she still fits in the Marathon, you could get a booster now and start training, and when she outgrows the Marathon or it expires, you can decide whether you think she's ready for full-time booster use.
Carseat-checking (CPST) and WAH mama to a twelve-year-old girl.