I live in Singapore and there seems to be no squashes, of the type that I typically see in Spaghetti Squash recipes, sold here.  Can I use pumpkin as a substitute? If I put it in an oven and use fork to make the spaghetti strands, will it work and have the same texture? 
You will not be able to follow a real spaghetti squashh recipe with pumpkin. If you try to gouge it with a fork, you will get grooved pumpkin.   My suggestion would be to use a spiralizer. Cut the raw pumpkin into pieces which are suitable for your spiralizer, and make pumpkin-spaghetti out of it. Then steam them. The taste should be pretty comparable to spaghetti squash, although you will spend more time in prep.  