OK, in full disclosure I have been dealing with generator auto start systems professionally for over three decades, and currently engineer auto start solutions for generators as part of my business enterprise.
In short...I do this for a living.
Generator autostart systems are used on every automatic backup generator. They come in a huge variety of configurations as part of the generator product lines of every majot brand of generator out there.
A modern day generator auto start or generator control panel does much more than just start the generator. It also monitors the generator paramaters on both the engine and the electrical side. Thinds like coolant temp, oil pressure, RPM, voltage, current and a host of other things are monitored, displayed locally and often times transmitted to remote panels and or alarm systems.
The main player in the retrofit market, and I say this because companies have come and gone are Deep Sea Electronics PLC and Murphy. Personally I like Deep Sea because of their wider variety of product offerings.
The industry standard to command a generator to start and run is a contact closure, with the generator control panel sypplying the circuit voltage and the device commanding the generator to run closing a set of contacts.
On a boat the best use of this would be to tie the generator control panel to the alarm output of a State Of Charge Meter. Ste the alarm trip to 50% battery capacity, and the alarm release at 90% or 95% and the generator will cycle the batteries properly. A great use of this is a boat with no shore power, such as at anchor or on a mooring.