Regarding the MS5351M as a possible replacement, I can safely report that I took a chance and ordered 30 of them from Chipmall.com when they still had them in stock.  The MS5351M is a Chinese version of the Si5351a that is apparently used in the Chinese space program so availability may be a bit better than the SiLabs 5351a.

My order for the MS5351M's arrived yesterday and I selected one at random.  Used it to build up one of my TSW Teensy 4.1 Raduinos for testing and I am very happy to report that the I2C address on the Chinese version is still at 60 so no changes are necessary.  Electrically everything tested just like if it were a Silicon Labs Si5351a installed in the Raduino and the alignment was identical.  I also noticed that there was no more jitter or other problems with the outputs on the limited test equipment I have here, but I compared the Chinese one to a real Si5351a installed on an identical TSW Teensy 4.1 Raduino.  Scope waveforms were identical as far as I could tell and the on air signal was looking good on the output waveform monitor (Elecraft P3 Panadapter attached to a K3S).

Jim, W0EB
TSW Project Coordinator