Can I do anything to make this electric piano work?
Yes. All you need to do is cut, swap and splice the conductors of the power supply. That's all that is needed to change a center positive to a center negative dc connector.

The alternative is to buy a modular power supply, like those offered by RadioShack with replaceable heads. You can simply choose the right size head and plug it in so that it is center negative.

As far as the current goes, the amperage listed on a power supply is the max it can supply. A device will only pull what it needs, so having a supply that can offer more is not a problem. As long as the voltage matches,  and the amperage is the same or greater, or should work.