There are NO GOOD reasons to leave a pet behind, only selfish reasons. Bringing to the SPCA means that you are going to kill them. Nobody gets a 4yrd and a 2yrd cat. Only kittens have a chance. Visit the SPCA site to confirm that. Better along a river but alive than dead,right? Plus they are spayed so they won't reproduce. Give them a chance, there are many places where to leave them free, they won't live safely but they won't be dead for sure.
Get a grip. You think leaving a domestic pet at the side of a stinking river is better than trying to find it a new home? WTH is wrong with you? How is it better to let them starve or be ripped to bits by feral dogs, than to die in a humane fashion in a vet clinic?
If you aren't willing to take them, do the decent thing and take them to the vet to be put down. You took on the responsibility when you took them in. Abandoning them on the street, or giving the SPCA the job of failing to rehome and then euthanising them is a major cop out.
It was the lunatic PP that suggested abandonment, not the OP who is trying to find a new home for these cats. I'd say the OP is well aware of what will happen to the cats if they are taken to the SPCA, and this is why s/he has not seen it as an option...
Will vets in Singapore put animals down if they are healthy? Just curious. In some countries they don't.