hack and build the controller.
it is US$100 kitchen tools and became a toy once malfuction, why so easy broken tools?

MN101C78A, mask rom MCU, no way to replace or repair this microwave oven as controller broad malfunctioning suddently.
Panasonic build the MCU and matsushita manufacturing, later nvuoton brand named.
see datasheet IC_datasheet

user manual, service manual etc. pretty much useless, not for component level of trouble shooting

LCD has no display, power supply and 18V is good, on-broad 3.3V is good, 8MHz xtal is good, reset circuit is good. so the only problem came to be fault of this MCU.

ON-OFF control for transformer to power magnetron, that the esential of regulated the power of microwave oven (old model without uses of inverter design)

the 18V supply, relays and buzzer
complete project log, power_broad

see tranining and why manual

this is no much more than a "timer" + relays to control for the microwave oevn, the following is reference, and perhaps good however the license was saying no any publish could be allowed as saying with an opensource code & project ?
https://extremeelectronics.co.in/avr-projects/microwave-controller-using-atmega8-avr-project/

another github hosted open source project that seems promising,
https://github.com/SamuelGold/arduino-microwave-oven

LCD1602, a few button and code, it is easy, ON-OFF control and timer setting would be only barrier for the project. let us see what would be.

complete project log, panel_fix

complete project log, plate_motor_fix

turns out that simple on/off control to power the magnetron, heat spreading and function of time is only way to control the heating level.
this is true for all kinds of non-invertor design of microwave oven.

https://www.facebook.com/laba.xiao.75/videos/6765868633453997/

bad design & unreliability of door lock mechanism, the fault of fuse burn or MCB tripped there is no way to re-design such door lock linkage or the swtich fastenning, simplest way is to remove the short switch to ensure no fues burn upon door open/close, the major drawback was that omitted safety protection for door opening when microwaving. User has own resiponsible to stop power before open the door.