Dixie chopper engine identification

I have a 66" dixie chopper withe a 33hp generac engine . I would like to know if there is any way to read engine castings to figure out year and size engine. Reason I'm asking is I had a new motor put on chopper 2 years ago . Today it started leaking oil pretty good all of a sudden . After closer investigation I noticed that motor had been tore down before ( since there was a half a tube of rvt used to put engine back together). I continued to search and started running serial numbers and model numbers off the engine plastics. The serial number didn't match the motor I paid for on my invoice it actually arched the original motor . Now I'm trying to see if I have been screwed . These motors come complete as a unit so why would they put my old plastics on my new engine..