+1 on what Bart said. Starting on a 600 is definitely wiser.

And to clarify... The MRA has novice and expert classes that apply to the racer, not the bike. There is no "amateur" status but they do run races that include both novice and expert racers - these are the amateur races.

As I mentioned, the novice or expert classification applies to the rider. Under those divisions are basically lightweight, middleweight, heavyweight, and open classes that apply to the motorcycle. The R6 falls in the middleweight class and can race up - that is to heavyweight and (I think) open. But you cannot race down, so if you were racing an R1, you can only race in heavyweight or open races.

As Bart said, there end up being more races you can run on a middleweight bike since you can ran up to higher classes. The rulebook spells all of this out but I hope this was helpful.