Hi Bojangles,
The numbers mean it is a 300 series motor. There were 2 versions of the 300 motors. The numbers from 350,000 and up means it was a kit and could end up on any bike. If the numbers are 300,000 and up it was a Whizzer produced bike & motor, IE Pacemaker.
The motor normally had a carter carburetor, and the numbers on the bottom of the cylinder simply tell which series of cylinder on the motor [it means the motor has the 7/8" valves]. The 300 series motor is worth more than the earlier "H" & "J" because it has 3 HP instead of 2.5 HP. Not sure what you mean by "Rochester" frame, but most had the Schwinn frame, but Cleveland welding was also a popular choice if it had the springer front forks. If you need NOS parts I have a large supply, and if you want cheaper reproduction parts I can advise you where to locate them.
The 300 series motor was produced from 1950 to 1951. The last motor series was the 700 series, and a 500 series was produced as a replacement motor around the same time.
Have fun,