What frequency does a microwave operate on?

Following international conventions, microwave ovens at home or in restaurants operate at frequencies of about 2.45 GHz , i.e. λ = 12. 23 cm. Figure 1 depicts a typical microwave oven (many details can be found in [1–3]). Microwaves are generated in a magnetron which feeds via a waveguide into the cooking chamber.

You will have to change the magnetron and change the capacitor for a higher capacitance . Most lower wattage microwaves have lower capacitance capacitors. Or you can ditch the Microwave oven transformer and capacitor idea together. You can replace it with a panasonic DC to AC high voltage Power Converter.

A microwave oven generates its radiation via a magnetron which is based on a resonant cavity. Unless the cavity has been damaged in some way, there is not really any way for it to change its output frequency .13 Eki 2020

The frequency is tuned to the cyclotron resonance of the electrons in the magnetic field , anywhere between 2–200 GHz, hence it is often referred to as Electron Cyclotron Resonance Heating (ECRH).