Hola Shirley

The most accurate translation of "tengo pensado un buen plan" is "I have thought of a good plan" (~ I have a good plan thought). This structure using "tener" instead of the auxiliar "haber" is putting an emphasis on the "completion of the action".

There is nothing missing in the English translation. We're using "of" because in English you use "to think of something" (that we don't use in Spanish).

Saludos

Inma