Look on the bottom of the locomotive to identify the maker. If it's Bachmann, forget it. Kato or Atlas have replacement motors available for some models. Not sure about Con-cor or any European makers (Arnold, Rivarossi, Trix). Let us know what the maker is and we can work from there.
As far as ID, can you post a picture? If not, how about a description? Include the # of axles, design style (streamlined or not?), paint job and markings. For comparison pictures, you can find thousands on the web, but it is a little hard if you don't know what to look for. HTH some.