If you want a cheap and fun track bike, get a mid to late 90's Japanese 600 or 750. You are close to Portland and Seattle so there are lots of opportunities to ride it. If you get a 600 and then later decide you want to race then the 600 gives you the most classes to race in. However the racing in 600 amateur can be chaotic at best.
I havn't checked to see if there are any Battle of the Twins or Sounds of Thunder classes there but if you get a V11 and set it up for track and were to decide to race in these classes you may not have the most competitive bike but most of the racers tend to be a little more "gentlemanly". It certainly won't be cheap but it will be a lot of fun.
I'm looking for a V11 that I can turn into a track bike someday. It may even qualify for vintage soon!!