IF dudduddud is correct in quoting 126°C as imposing the upper speed limit (I have no way of knowing otherwise), AND there were no other CAS / Mach No. limits, -
At and above the Tropopause (11000 M / 36089.2 Ft) and at ISA Temperature (-56.5°C), a TAT of 126°C translates to M 2.0522 and a TAS of 1177.3 Kt. The associated CAS will depend on the particular Flight Level, and as a sample -
F/L 450 = CAS 716.3 / EAS 518.0
F/L 500 = CAS 650.7 / EAS 459.4
F/L 550 = CAS 589.1 / EAS 407.3
Temperature deviation from ISA will, of course, change these values.