If it is an older sporran, it may be hard, if not impossible, to replicate the missing tassel and cord. The better route may be to re-position the two remaining tassels so they divide the front of the sporran in thirds. Many old horsehair and goat sporrans only used two. I agree with Bren, the only way to know is to consult with a sporran-maker with the the supplies, contacts, and skills.