Energy harvesting circuit modification
The circuit shown using the MAX1675 is a boost regulator producing 3.3V and if you look at the data sheet, it is capable of being modified to produce up to 5.5V. Page 6 says that if the FB pin is grounded, the output will be 5V.

This is dependent on sufficient voltage from the piezo device and loading conditions on the output being reasonable and compatible with the energy you can take from the piezo.