Adding an automotive starter isn't a big issue, I did that to my S-o-M with its replacement ST head generator: Simply bolt a flex plate to the gen head pulley, fabricate a bracket to hold the starter motor, and that's starting sorted.
You don't actually need to operate the decompressor, if you've got a solenoid capable of pushing the fuel rack shut & holding it, you can stop the engine without decompressing. Any auto starter motor should be capable of pulling an engine through compression as well.
To my mind, the biggest hassle is the rack operation: You want a "fail safe" system which, if all electical systems died, shut down the engine via some kind of mechanical movement; which means something spring-loaded with sufficient travel (either direct, or via a bell crank) to allow the full normal rack movement, and able to work against a spring that's sufficiently strong to overcome the governer's ability to pull the rack open again... If I didn't mind losing the failsafe option (or having some kind of battery-operated emergency backup), I'd have a coarse-thread screw on a geared motor, set in line with the end of the fuel rack. Screw comes out to stop the engine, goes in to allow it to start, much like the COV.
Speaking of COVs.... Lister never addressed this with the 5/ or 6/1, or the twins... if you're loading your engine to max HP you're supposed to switch to low-compression mode once the engine is spooled up... but this HAS to be done manually, and it has to be reversed after the engine stops. I guess there are three options here: Use a COV-less head (ala 8/1 engines); use some kind of screw in/screw out device, which was used on direct coupled 9/1 (JP1) gensets; or just ignore it as I'm sure most Lister engine operators did... Personally I'd fabricate something which spun the COV into low compression mode after the RPMs hit maximum.
Thinking more generally, Lister put 24vdc into the 240AC circuit, and used current draw as a sensor. Whilst that's fine on a lighting circuit full of incandescent lights, or even with more complex devices like valve radios and possibly even some TVs... I do wonder if that's still a practical option these days? e.g. if there's 24vdc on the line, would switching an LED light on take enough current to be noticed by the SOM? So the next step, I suspect, would be to combine the genset with a medium-sized battery bank, and an inverter of an appropriate size for the circuit. Use the inverter/batteries to provide "instant on" 240AC (110AC as applicable), and only fire up the generator if the batteries need charging, or of the load goes over a certain level. Which also makes me wonder if a 240AC generator is even necessary (or desirable), if you can get a lower voltage version to charge your battery bank.