I've always understood that preformed ponds are the devil to instal as they have to be bedded in on sand and absolutely level which is hard to do. I also believe that the fibre glass is prone to cracking in hard winters though the technology may be better now.
I'd be inclined to buy a paddling pool or large plastic bin and put the goldfish and any plants in those then empty the existing pond and mend or reline it with a sheet of pond liner or, better still, butyl which should last 30 years.