Any adapter which has the same output (volts, amperes), connector and polarity could work - inside the instrument there are regulators so I think they would deliver proper voltages to internal circuits.
Maybe yours will work too, when you will set proper voltage +5 Volt with VR1 in regulator circuit. -5 Volt rail seems to be allowing more tolerance - according to the point 1 it can be between -4.7 ~ -5.7 Volt. Just try it. If voltages will be out of spec, you should try different adapter, more near to the original one.
Daniel Forro
Few steps in adjustment procedure consists of measuring voltage while using original 9V, 300mA AC adapter, however when i bought the synth i got not original power supply - its 8.5V, 350 mA. Do you think that i could use that one for this procedure, or do i need to buy specificly 9V, 300mA AC adapter? If original adapter is needed, maybe you know some alternatives for it?