No problem! There is no such thing as a stupid question if you are being sincere!, Most of us advanced Windsor troopers are more than happy to help you out!
The stock system is comprised of an in-tank electric fuel pump rated between 88-92 litres per hour, into fuel lines to your motor connected to stainless steel fuel rails that feed 8, 19 lb injectors, regulated with a fuel pressure regulator, there is a thing called the schraeder valve before the rails to check pressure, this is also the spot that you can permanetally install a fuel pressure gauge as well.
The answer to your question is YES, your 90 comes with a FPR stock from the factory, BUT it is not adjustable.
Changes between stock and aftermarket? what do you mean?, I think you mean as far as settings. The difference is only that the aftermarket is adjustable, usually with a hex key provided, or some other manner. You can still run an "adjustable" regulator at "stock" settings, which is 38-42psi at idle with the vacuum line disconnected, in other words, you disconnect the line, make your adjustments, then reconnect the vacuum line.
If your car is dying, it is most likely something else, you need to invest in a inexpensive EEC scan tool ($35) and do a complete tune up first, I suspect faulty sensors or just plain age.
Try not to put the cart before the horse.
Ever replaced the oxygen sensors?