Thanks for the feedback on the Si5351 as a replacement for crystals.
It would appear that a low cost micro-controller has to be put alongside the Si5351 synthesiser. I wonder if anyone has a complete solution designed and tested. Also, what about programming?
It would seem that there is a real need for a simple KISS way to replace crystals, so I'd be surprised if someone has not already done this. According to Peter Marks the controller should only be a few dollars. This feedback (that you have to tell the Si5351 the frequency every time) is also what Jay told me.
It looks like a total solution that also replaces the oscillator should be possible for under $10.
Please let me know of any links.