You won't get a 'less clunky' version - it's a 9V AC output which will have to be derived from a 'clunky' old mains transformer even if it's a 120V AC input version.
Such wall warts should be available but won't be too common.
What you could do is check out the input circuitry of the synthesizer to see how that 9V AC is being used. I suspect (needs confirmation) that it will feed a bridge rectifier to deliver a DC supply internally. If this is the case you could probably replace the 9VAC adapter with a DC output power pack (the internal bridge rectifier will sort out the polarity issue).
But this needs proper confirmation - post a picture of the innards of the synth, around the power plug and associated circuitry.