Well if it was long enough, I would braid it if it was a hack class. But if it was a mountain and moorland class don't touch it, groom it yes but that's about it now if it was a western pleasure class, you either band it of leave it out but the mane is either banded or hogged (shaved off)....but again it all depends on what class or event you enter
Take some hair from the tail, create a falsie. French braid what you have and then sew falsie to it.
If done correctly you can't tell.
I can also do false manes on M&M's and hair extensions on Traditionals.
Last 3 plaits in this mane are fake as the pony has a damaged mane and
Permanatly has only a few wisps of hair. (please ignore my riding in the above photo and him overbending, this was many many years ago)
Bottom half of this ponies mane is fake due to sweet itch