You can ask on your local freecycle group for amplfied PC speakers - on ours they come up quite often.
Most come with a lead terminating in a 3.5mm jack plug. if the jacks on the keyboard are 6.35mm (quarter inch) then you need an adaptor like this:
http://image.shutters...ckground-20617879.jpg
maplin and the like sell them, and you find them included with headphones from the pound shops and market stalls.
On the subject of the amp, it is probably fixable with off the shelf parts - in my experience, at most you 'll need to replace a couple of capacitors and possibly an I.C. or output transistor. Any decent tech should be able to sort it out, try looking for an old school TV repair shop in your area.
Try also plugging in and disconnecting the heaphones a few times - in mnay cases, 'phones mute the internal speakers - the switch contacts may be dirty and doing this might clear it....