Provided you study the operating manual and comply to any specific indications above all, in my experience the Rotax 505 allowed me to fly during the winter in northern Italy (take-off at little more than 0°C), and I was once able to go see the panorama from 5000 m AMSL with no problems (estimated 25°C below Zero). Of course the engine was producing reduced power and climb at altitude.
Starting the engine when cold-soaked after a prolonged wave flight is probably more trouble prone, but still possible. Battery performance matters, as well as engine conditions. Generally, having used the engine for self-launch or a test-run a few hours before, allows it to be still that little warmer and start more "reliably". Apply power progressively. Generally, less than a minute is enough, the engines have a small mass and warm up rapidly.

Currently I own a watercooled two-stroke SLG, and the manual mandates to warm the engine to 40°C before take-off. Takes maybe less than a minute while checking circuits and emergencies.

Aldo Cernezzi
www.voloavela.it