My yz426 has the same problem. I will be running a heat exchanger this year in lieu of buying the radiators you picture. Although those are nice units for me a free exchanger from a dead sled is the way to go, but I think those will solve your problem. Mine runs ok on the snow and at many times won't even warm up, but a few minutes sitting in one spot on the trail or stuck or screwed up and that thing wants to boil. I hope to keep it cooler overall with the exchanger and a thermostat to help it warm up on those cold days and on the way by I hope to catch the carb with some coolant to heat it too! I'm sure the exchanger will require scratchers come spring and may be a problem on dirt conditions. Think how much fun it will be finding out! Also try removing your fender that helped me alot!
