I do not know if this has been posted on this forum before or not so sorry if this is a duplicate. This guy has several vids about a custom counter balancer design he did for a cr500 that he put in a trx250r frame. Watch his vids 6-10. I do not like the required frame mods for this design but it is interesting.
http://www.youtube.com/watch?v=dS_jT8TdHGE&list=TL7ddpHxsSQawThanks goes to Kevhes11 for the link.