If it's the bolt, just visit a parts store and get a replacement. If it's the drain itself (which the bolt shouyld definitely strip first), well that's a bit bigger problem. I think there are pressure plugs you could get... Not sure.
Comes in a little kit with a tap, maybe a drill bit, and a 5 or 10 of these "insert" things. Costs maybe 10 bucks or so. It's been a while since I bought one.
You use the provided drill bit to drill out the old hole, also removing all of the old thread. Then you use the provided tap to tap in new threads (which will be bigger than ones you just reamed out). Then you thread one of the inserts into this new thread. Et le voila, vous etes done.
The insert is nothing more than a coil of metal, looks somewhat like a coil spring, but I think that it has a square cross-section, not a round one. Anyway, who cares, the outside of the insert threads into the new bigger hole you just threaded, and your plug threads into the inside of the coil, which provides the same diameter and thread as what you just drilled out.
I've had one for the exhaust pipe on my motorcycles front cylinder for 15 years now, and both of the spark plug holes for about ten years, and they've been no problem. And as you can imagine, spark plugs and exhaust pipes are heavy-stress uses.
One downside for you, of course, is that all the metal shavings you're about to get from drilling will fall inside your tranny, so you'll have to get them out somehow.
Also, being that the tranny is probably still strapped to your car, you may have to drop it out just to get access for drilling and tapping.
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