Non-certified parts for a certified plane?

Hello,

Do I need certified engine monitors for a certified plane if I keep the
factory ones installed? I was asking this to someone at our airport
today and they thought that I could install, for example, one of those
fancy engine monitors you see in experimental airplanes, as long as I
had the certified counterparts that are required (egt, cht, fuel flow,
etc). Is this true?

Also, for non-essential flight gizmos like fuel flows (for example)...
why do I need a certified fuel flow compared to an non-certified one?

Thanks,

-dr