I remember asking this same question here after buying my first MEP genset a few years ago. I didn't fully appreciate a recommendation provided by another SS member at the time to "buy a 2nd unit" as the ultimate spare. So I started procuring piecemeal spares for my genset...a spare starter, fuel pump, injectors, AC voltage regulator board, etc. It soon hit me that I was spending enough money on what I considered potential failure points that I was nearing the investment necessary to buy a complete 2nd generator if I shopped around wisely.
The problem with having various spare parts is that you never know what part might fail and stop you dead in the water during a critical power outage situation. That exact situation hit me during a week long ice storm power outage. My main house genset had a major electrical component failure causing output AC voltage to go high. Thinking how prepared I was, spares wise, I thought for sure it was my voltage regulator board and I fought freezing cold and nearing darkness and installed a spare VR board standing next to a silent dark house. However it was not my voltage regulator board and the spare VR board I replaced it with did me no good. Fortunately, I had eventually taken the other members advice and picked up a second genset at a great price a few months before the storm and had adjusted and load tested it earlier. So all I had to do was swap the cabling to my backup generator and our house was powered up and heating before my wife returned home from work. Otherwise, she would have driven up to a dark cold house and seen me outside freezing and swearing at a dead generator!
By no means am I totally discouraging picking up obvious spares. However, where does it stop and how much does that cost to cover all your bases as far as potential failure points go? Even if you have the right spare on hand how long will it take to determine where the failure lies during the added stress of a power outage? You can read on various threads of it taking days to weeks to trouble shoot a problem and implement a fix. At some point its wise to consider a backup to your backup (the saying: 2 is 1, 1 is none came true for me). So, if you can find a serviceable non-working generator same model as yours at a great price and have the time to fix it you will be way ahead of the spares game since you have 100% spares with redundancy! Having a 2nd working genset gives you peace of mind knowing its there for you in time of need.
Edit: Even if you can't economically fix a non-running spare due to a really bad problem (broken piston, cracked block, etc.) your still left with a parts donor unit with 99% spare parts that you might eventually need to make repairs on your unit.