The 685 is proven big bore based off the 600. There are a few companies out there that build these motors but the best and most reputable (imo and many others) is Carl's Cycle in Idaho. It is a very quick reving, snappy, strong motor. My 685 is carbed and it is solid motor. Fbomb can attest to its durability and strength. It will take down many bigger CC'd sleds. It is typically described as being equivalent to a well tuned, single piped 800.
Hope that helps.