Needs some experimentation. If two cores are mated there is still a small gap plus chamfer taper of effective cross section at the meeting ends so we can't presume exact equivalence to a single seamless length of powdered iron.
The width control has to be able to compensate for variations in DC HV to the CRT (changes deflection factor), variations in flyback sweep voltage, and variations in the total of the 10 uH linearity coil plus the nominal 80 uH yoke (mine measured 80 uH). I suspect .75 inches long alone of the permeability 20 substance would work for me but is likely insufficient for accommodating all variations in the SE/SE/30 installed base.
Maybe a suitable repair kit would be two .75 inch long cores of the 20 permeability, one to replace the most instances of broken core, and the other, just in case a further boost is needed in inductance, or in case of mishap with the first core.
I'd avoid the teflon coating: we've had enough experience with clumping already! Some sweep linearity experimentation may be necessary between the permeability 20 and 10 choices, my hunch is that u=20 and .75 length are a starting point, with two cores on hand in case a second is necessary. Even if the second is way longer than needed, most of it will be so far from the coil I think core losses will not be an issue. Would they send you two free engineering samples of u=20 and two of u=10, all in length .75 inches?