My Magic Chef gas oven thermostat is not working properly. At 400 degrees it only heats to 200 degrees and at 500 degrees it only heats to 300 degrees. Do I need to replace it or can I calibrate it?

Ther is a temp sensor that goes from the control valve to the oven,this may be the problem, make sure sensor probe is tight where it screws into the gas valve and other end is not just hanging in the oven, it attaches to the top of oven,if it's where it's supose to be, I'd change that first

Well after checking the model # u listed cgr3725 there are 4 sub models from that.. all appear to have the same components for oven controls.
you will not be able to "calibrate" 200 degrees out of the control, either the temperature probe has been damaged, the control is bad, or your ignitor is just plain getting weak and not pulling the required 3.3-3.5 amps thru it to allow for full flame height and normal temps and heating times.. you would need to test the current(amperage) flow thru that ignitor to confirm its failure.
good luck, and have fun..
ttfn