It's important to note that altering or using an incorrectly sized microwave plate could potentially lead to safety hazards. Microwaves are designed with specific dimensions and materials to ensure proper functioning and containment of microwave radiation.
Using a replacement plate that is significantly smaller in diameter than the original could potentially result in uneven heating or create gaps through which microwave radiation could escape. Microwaves work by emitting electromagnetic waves that are contained within the microwave cavity. If there are gaps or improper fittings, it is possible that some radiation could leak out, which could be harmful if exposed to it directly.
For safety reasons, it is generally recommended to use the correct-sized replacement parts for microwaves, including the turntable plate. I would advise you to consult the manufacturer's guidelines for your specific microwave model or contact the manufacturer directly to inquire about the safety of using a smaller plate. They can provide you with the most accurate information and guidance regarding your specific situation.
To ensure the safety of yourself and others, it is best to use the appropriate replacement parts recommended by the manufacturer or consider getting the original plate repaired or replaced with the correct size.