You can try cleaning up ('chasing') the threads with a bottoming tap like the one below:
https://www.ebay.com/itm/M6-x-1-00-HSS- ... SwHjNWAGS1Your hardware store may have one that's similar, but make sure you get one with a blunt tip (bottoming) like the one above, and not the standard pointed tip. The store should also have a tap holder, which you'll need to drive the tap in straight. You didn't mention the rear bolt hole threads, hopefully they're just rusty too.
This tap should clean up the rusted threads easily, just use some oil on it first, and clean the hole out well after. This will also give you a feel for how much resistance the tap has with undamaged threads.
With the damaged threads, I'd measure how deep the hole is first (with a short rod, Q-Tip, whatever), then oil/grease the tap & run it straight into the hole as far as possible until you start to encounter greater resistance, back it off 1/2 turn, turn it in a whole turn if possible, back it off 1/2, etc. until you've gone as deep as you're comfortable with (and hopefully nearly as deep as the hole; the blunt tip allows you to go extra deep). If it starts getting real hard to turn, do NOT force it, just back it out and call it a day. Then, you can use an extra-long stainless bolt that can be screwed in deeper to engage the new threads you've cut (and hopefully also some damaged ones you've cleaned up). A stainless bolt can be shortened if needed without rusting up afterwards.
If this doesn't work, the hole will probably need to be drilled out and a Heli-Coil insert installed. I've done this many times, but IMO the above is definitely worth trying first, you might get lucky with the damaged threads.
Ray