It happened to me last year. I just had to open the case, remove the old switch, and take it to an electronics store.
It is called "pushbutton" power switch. You can buy a new one, of the same type (there are a few different models).
Then all you have to do is the replacement. If this is not your field, let it be done by an expert (won't cost you much, and it will be brand new again!).
Pay attention to the high voltages (especially near the CRT, and the power capacitors). Also, you may add the special isolation (some sort of thermosensitive plastic wrapper over the switch/cable contacts), in order to avoid any shortcircuit.
That's all ;-)
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Subaru r-2 specifications