Seeing a thread like this always stings because I went through the exact same thing a couple years back. Mine got stripped while parked at my old apartment. Handlebar gone, every wire and cable sliced clean through. I had almost no money at the time so I figured it out the cheap way and it honestly turned out fine. Start with the parts hunt before anything else. Stock Grom bars and controls show up used constantly because people swap them out for fat bars the day they buy the bike. Check your local classifieds and any Grom owner groups on Facebook. I got a full set of controls, mirrors included, for under a hundred bucks that way. The wiring harness is the scary part but you don't need a whole new one. Get a wiring diagram for your year, it's floating around free online, and splice in new pigtails for each cut line. Heat shrink every joint and route everything the same as stock.
For the brake line, if it was cut at the banjo you can sometimes just replace the line itself rather than the whole assembly. Bleeding the front brake after is fiddly but doable with a cheap one-man bleeder kit. Take your time with that one, a spongy front brake on a little bike is no joke.
Also worth asking your complex manager if any neighboring buildings have cameras facing the lot, even angled ones. Mine caught nothing but the guy two doors over had a doorbell cam that got a partial plate. File that police report update with whatever you find.