Parts which are no longer manufactured are replaced with automotive parts. Much aviation technology is rather dated and unreliable, in general, because certification excludes newer technologies. Peak GA technology was the early 1980s. The only exception is large turbines. Think about how much engine and metallurgy have improved since the 1980s. There now exists parallel economies. Those certified and those which are not. The uncertified versions cost less and are more reliable, in general, than the certified counterpart. Aviation exists entirely outside of free market economics. In fact, many times the non-certied equipment is used to fund certification. You need to keep in mind, FAA certified anything lives outside of free market economics. The result is, the non-certified variants have become more reliable because iteration is cost effective. Iteration is contrary to the certification model. Certification doesn't mean safer. In general it means massively more expensive for ancient technology whch isn't modern because it's cost prohibitive to do do. Honda (iirc) developed a new series and generation of piston aircraft engines. Twice as reliable and about half the fuel consumption of current generation (1970s-1980s tech). They started the paperwork for certification. The FAA made clear they could never be profitable because of certification (certification would be several million per engine and they developed an entire line for every piston segment. Whereby they would then require per aircraft type certification or exemption) They intended to significantly under price lycoming and others. But quickly figured out they could never be profitable because of certification requirements. They canned their entire engine project (all internal development was complete) even though their own testing confirmed cheaper and more reliable than current certified flavors. The FAA doesn't exist to keep the skies safe. They exist to maintain a cost prohibiitive barrier to entry against captured market competition.