The Zenith carbs for the 248 are much more common, and much less expensive than those for the 270. Not much available (ever) that is better than the Zenith. I would suggest, if you wish to replace it, find another Zenith with the same tag number (assuming that the Zenith you have is correct for the application).
WD-40 is probably not going to free the frozen shaft. Remove the throttle body from the rest of the carburetor, and remove everything you can remove. Now, heat the throttle body as hot as you can in an ELECTRIC toaster over (DO YOU USE A GAS OVEN!). Remove the evenly heated throttle body from the toaster over, and with an acetylene torch, spot heat the casting where the throttle shaft goes through until cherry red on both sides. Set it aside for an hour or so and allow it to completely cool naturally. DO NOT QUENCH IN WATER!. Now, gently try to work the throttle shaft, using the WD-40 as needed.
Jon.
