"Bill" wrote
Don't forget that there are operational FARs like you quote and
airworthiness FARs. For example, if the instruments are converted to all
electric, then look at 23.1331 Instruments Using A Power Source - For each
instrument that uses a power source, the following apply: (c) There must be
at least two independent sources of power ... and a manual or automatic
means to select each power source.

The original poster's airplane is certificated under CAR3 and not FAR23 and
so 21.101 could be applicable if this modification was performed by STC.

However, no STC is required to perform the modification, so 21.101 is
inapplicable. 23.1331 is also inapplicable, and you will note that
most airplanes currently in service do not meet it.

In other words, none of this means anything.

Michael