I need help to find the serial number on my 28 plymouth roadster I have the tag number on the drivers side can't read the medallion and the motor number starts with C the tag on the floor reads R-4103-0
The FedCo medallions can be very hard to read. For a 1928 Plymouth they should start with "HL", "RW" or "GP". That will be your serial number.
The engine number should start with the engineering code for the car which in 1928 was "Q". I am guessing that your engine with a "C" might be a replacement of some sort.