I had a friend that had a major hole in the bottom of their FG pool. They ended up lining the bottom with sand and intalled a vinyl liner in it. It actually turned out pretty nice. They screwed the channel around the top to accept the liner bead and used the channel assembly that usually goes around the steps on their steps. I believe it cost them around 2,800 for the whole "remodel". The tricky part is getting the measurements right.

Their hole was bad enough that a couple of contractors told them they should just fill it in. It may be an option for you too if yours is very bad.

Good luck.