A cheap deal thermostat is about the most reliable part on the racer.
It doesn't care what temperature the coolant is when it arrives, only when it leaves, and all automagicly.
A restrictor is a guesstimate at best and won't follow the needs.
I found that preheating the oil before starting the motor was a waste of time because as soon as it hit the cold block the heat is gone any how.
In the case of the standard shift drivetrains, the hot motor oil is run through the tranny before returning to the supply tank, that improves shifting, reduces the power loss, and sheds some heat along the way, kinda like those pesky bike motors.