Stupid idea
Older cars are not designed as stop/start cars...
Cars with Stop/start have loads of sensors for knowing when it ok to stop there engine and when to restart it (One the main things being battery voltage level but there a whole list of other stuff they check)
Plus am not sure if a normal starter motor and flywheel ring gear could even stand up to everyday stop/start as they where not designed for this amount of usage..
How would a driver know when his battery going to go flat to start while he has the lights, stereo, heater, heated seats, GPS, dashcam, heated windshield, Heated rear window etc etc is on