you dont need a new syphon, you just need to replace the polythene disk that lifts the water to start the syphon. There are different shapes and sizes of disk, take the old one to your plumbers merchant for the correct replacement.
you will need to remove the syphon assembly to get it out, it's the only way.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.