The issue is the fuel quantity. You can replace your MP, RPM, etc with a TSO'd or FAA-PMA gauge with out any issues. It's my understanding that the 930 is the only unit that can be installed in your aircraft and be the primary instrument for fuel quantity and everything else.
Wait, whoa, no, that is not accurate. Mooneys are certified under the original Type Certificate, plus a Supplemental Type Certificate for each model and/or serial number range. The Type Certificate/STC is specific down to the make, model and part number of the components that are required equipment. A new instrument that is TSO'd and is the same make and model as the replaced instrument, can be installed to effect a repair. However, the required equipment instruments cannot be substituted for with something else, unless the substitute is STC'd. Manufacturers of aftermarket equipment such as JPI need to get an STC that lists the specific makes and models of aircraft into which the aftermarket equipment can be installed in place of required equipment.
http://www.airweb.fa.../2A3 Rev 52.pdf
I warn you, it is a long document and convoluted reading.
Most panel instrumentation for engine functions in a Mooney are required equipment. In my aircraft, which is a turbo (231), this includes TIT, RPM, MP, Oil Press., Oil Temp., the factory EGT and CHT, and yes fuel quantity and I believe fuel flow. Probably some others I have not remembered. None of these can simply be replaced by a TSO'd instrument that is not listed on the Mooney Type Certificate, unless the instrument is STC'd. You could not, for example, take out your MP and RPM gauges, and stick in a couple of replacements of a different make and model that are not STC'd, because the aircraft would not then conform to its Type Certificate.
A major difference is that instruments that are within the Type Certificate generallly are part of a system, that includes things such as a redundant readout, or a warning light that tells if the instrument is malfunctioning. To be STC'd, substitute equipment needs to have proven redundancy/warning systems, and not just the instrument function served by the instrument substituted for. The 930, as an example, has a Remote Alert Display, a small digital backup readout that must be installed with the 930 in order for the 930 to be primary for engine functions.