You can usually just replace a power button with a new button. There will be a set of cables that plug into the motherboard that will also need to be replace along with the power button.
Without taking your receiver apart there is no way to know if the switch can be replaced with a generic one or you need to get one from Sony (they are not great with parts for older units). A local repair shop should be able to help.
Hi
The simplest way if the receiver will turn on and stay on would be to fit a switch in the power cable,you can get remote contolled units that plug into the wall and your receiver into that .