assuming that engine is non-interference and a timing chain breaks -- is only replacement of the timing chain necessitated or would a whole new engine be needed? -- aside from mileage -- is there anyway to tell if the timing belt is ready to go?

 

CX-3 has a timing chain, not a timing belt. They are designed to last for ever. Signs of old age or wear are abnormal noise & often fault codes for Cam / crank correlation.