Voltage regulation with a constant power
Use a very efficient buck converter.

Power In = 12V*0.2A = 2.4W

Power Out = 5V*0.45A = 2.25W

Efficiency = 2.25W/2.4W = 0.9375

So, ~94% efficiency is required.

A very common LM2596 won't reach your requirements but gets close.

Some regulators such as MAX17502 claim values that make your needs look viable:



If you can relax your requirements down to 85%~ you will get many more options. Or, if you can determine how long max load will be required vs how long your 12V PSU can tolerate overcurrent you may get away with it.