Hi Riley. Here is a website that may help you identify your engine. Start looking in the section called "production engines"

http://www.mh-aerotools.de/airfoils/cox_frameset.htm

Be sure to compare the year your uncle thinks the plane was made, look at the number of exhaust ports and all other details because many of the Cox engines are pretty similar.

The same website also has information on different models (maybe your uncle's biplane). Take a look at the section called "Model Database".