A suggestion regarding home sized diesel generators.
These normally start from a 12 volt battery (large units are more often 24 volts starting)
If you have such a diesel generator and it fails to start readily, dont forget that you can normally use jump leads from a car with a running engine.
Some people only think of jump leads for starting other vehicles, not generators.

Double check that the vehicle battery and the generator starting battery are the same voltage.

Another last ditch option for a portable generator that wont start, is to take it to someplace that has line power or a large generator available. Use a line powered charger/engine starter and allow it to warm up.
Once taken home it should start readily whilst still warm.

Both the above are are a lot of faffing around, and it would be better to keep the engine and battery in such good condition that there is no question re starting.