It's powering on but heating food

This is most commonly the result of the magnetron that is no longer functioning.

But there are some other possible components worth inspecting namely the fuse, power diode, capacitor, transformer, etc.

On mine it was just the magnetron connector which had the small rf suppressor capacitor shorted out to ground. A u/s magnetron was obtained from a domestic repair shop and I just swapped connectors (which are cheap to buy) but not so easy to replace.

Please Note: If unfamiliar with electronics/high-power electrics, it is not advisable to fix it yourself. Lethal voltages are involved via the main capacitor and if not properly discharged can cause serious injury or a fatal electric shock.

Therefore, testing of components and experience in component replacement/ modification is a pre-requisit for this kind of diy job.