Getting a Garmin Zumo is an expensive option & they're fiddly to use unless you've used them before. How about a cheap 2nd phone? Download the Waze app & you're good to go. Plenty of handle bar phone mounts avail, ball mount ones are better. The phone could be charged up off the Aux socket under the screen, you'd need a DIN USB adapter plug for it though. (or you can go off the wires from behind the DIN socket)
If you do go the Zumo way, you can use the power behind the Aux bracket (or buy an appropriate adapter to fit in the DIN socket that matches the Zumo)
You haven't left yourself much time to get it sorted really, another option is possibly borrow someones setup that just plugs in to the DIN socket?