Radio Shack has a prebuilt speaker amp that runs on a 9v battery. (At least they used to) Check
www.mcminone.com for kit amps. The problem you will face with this device is feedback to the mic input. The speaker should have a decent output as it is but there are two problems you might be encountering. First, is there a headphone jack? If so, it may be intermittent and needs some contact cleaner and exercise. The second is the speaker, these do make a much higher output when they are in a box, or "infinite baffle". With nothing surrounding the driver, the out of phase back radiation is mixing with the front.
What did this come out of? It is possible that the signal routed out to another spot on the device and all you need to do is add a jumper to route to the speaker amp.