Knotted String as Replacement for Toothed Belt
I’m going to recommend not using a fishing line with knots.  Probably the biggest problem you’ll have using the fishing line with knots is if the knots are not perfectly spaced,  movement along the X or Y axis is not going to be consistent. This could result in weird deformations in your print.

Depending on how you tie the knots they may not grip the teeth of the gears quite well enough to prevent slippage. 

Both of these issues will mean that you will not get very good quality prints assuming the print doesn’t outright fail.

You would be better off waiting to get the correct belt then attempting to use fishing line.