How to generate a continuous clock from one that periodically turns off?
Simplest one I would think would be to create a ring oscillator that has an R/C time constant where the R value is actually created by a transistor that is being driven with local voltage. Basically set it up so that you're phase locking onto the remote oscillator while it's up and running and when the remote clock goes down, you keep your local oscillator going without changing the last known voltage that drives the transistor/resistance. As "simple" as this is, you're still creating/using a PLL.