I believe there is one actual crag on the island, but there's def a couple of indoor (well plywood) walls.
I think there's a mountaineering club in Singapore. To be honest flights out to Krabi can be had pretty cheaply. That's your best bet. There's a lot of land to explore what with Vietnam, and Thailand. I haven't heard anything about Indo, but you could find a new Stanage?
It's a bit of a strange place Singapore. It's cool, very cool, but very weird. The entire country being one big city. Public transport is cheap and easy though. Very, very easy!