My understanding from some cursory research (I'm not an expert on Keirin racing!) is that to be a true 'Keirin' bike, it has to be made by a certified builder of specific sized tubes and lugs so, you're right, no one in the US is likely certified and therefore it wouldn't be a Keirin bike. However, replicating the geometry and particular fitment requirements of a keirin type bike shouldn't pose any builder a challenge. I have made several framesets for local velodrome racers and the velodrome Juniors Program and some are using NJS parts. I don't think there are any specifics that most any builder couldn't meet. I'm now curious and want to go do research into these certifications (which I really should do before commenting on them!).

You don't mention how tall/heavy you are or how you ride your bikes but the limitations of a true Keirin bike may actually be your problem. You say you've broken several in the past year. My guess is that for a Keirin racer who makes money racing, replacing a frame every few crashes isn't a problem (as with all high level racing) however that same rate of failure isn't very useful for the rest of us. The specific rules for Keirin are for a standard diameter tubeset which is a bit outdated. As TiHabanero mentioned, I made a frameset for his son from a special set of 70's Columbus PS track tubing (which is standard diameter ). Ti's son is a tall, strong guy and 1" top tube and 1" steerer aren't sufficiently strong for him to race on. The top tube had also previously been nicked by a saw at some point in the past 50 years so it had to be replaced anyway. As the goal was specifically to use this tubeset, I used the 28.6mm DT for the TT and used a new DT with appropriate diameter and thickness and then increased the headtube to 1-1/8" to stiffen up the whole front end. Those tube diameters also require different sized lugs that wouldn't meet any NJS rules. This may take it too far outside of a 'Keirin' bike for your taste but it may be required to support your riding style, volume and type of riding you do.

The geometry of a Keirin racer is the same as a typical track bike (or close enough) and there are many builders of those and most would adjust the geometry to suit your needs - within reason. There is no reason to stick to the outdated dogma of a controlled racing series that requires a bike that isn't suitable (as you have proven) for normal riding. We have access to much better materials and techniques in the real world that you should take advantage of.

Having said all of that, I actually have a few full sets of Ishiwata (pre-cursor to Kaisei) 019 tubing that are just sitting here gathering dust.