The problem with anything electronic these days is the components have been so highly customized that there usually aren't any off-the-shelf replacements. Any electronic component that's more than two years old has probably been declared end-of-life already and replaced with something else that is entirely incompatible. So finding replacement parts is almost impossible even if you knew which one needed replacing.
The good news though is the price of almost anything electronic has been dropping like a rock for years. Finding a new microwave to replace that old one usually means just finding one that fits in the space available, and the added benefit it the new one will come with more features (bells and whistles).
This is a hard lesson that I still haven't managed to learn about computers and TVs. I always try to repair or upgrade them and in the end spend more time and money than it would have cost to just replace them outright and trash the old one.