The thermostats usually installed on Rotax engines are only useful to get 70ºC quickly on cold weather. But since they open the circuit to the radiator at only 70ºC, you still have the same problem to get temp over there.
The duct tape "solution" works, but is no controllable at all during flight. I've had to land several times to remove part of the duct tape because in every climb I was getting close to red line. Other times I should have put more duct tape and I've made the whole flight with only 60-70ºC. The advantage of Evans is that keeps consistently a good temperature.
I would recommend using the 50/50 coolant only if the aircraft is equipped with cowl flaps actuated from the cockpit, to have full control of the engine temperature in changing conditions.