Very fine fishing line works well and you can glue it with superglue. Glue one end and let it set fully. Slight stretch the fishing line and secure with a drop of cyno and hold in place until set. then trim the end. I used piano wire for the samurai bow tapered down in a grinder then bent to shape. It has a slight spring to it so keeps the string under tension. Here is one I did on my mounted samurai. Only issue I would have is the bow is bent a bit much for not being drawn and should look a bit more like the picture below. Google "yew bows" to get an accurate shape to follow and also note the colouring as yew bows had a distinct colour difference between the heart wood and the lighter sapwood on the front of the bow. I think it would be well worthwhile replacing the bow as it is a beautifully painted figure and an accurate bow would take it to the next level.
Bob