The previous owner wasn't much of a mechanic and amongst many other stripped bolts, I found the oil drain hole with barely any threads.
I thought about helicoil, time-serts, etc. but I ended up drilling it out and retapping. Looking in the case, there's not much material there so I don't think an M14 would work. The next closest size up is Imperial but I went with that anyways. I used 1/2"-20 tpi and I have no problem torqueing the drain bolt to spec (20N-m I believe). I used a 1/2" x 3/4" bolt for my drain plug. I had some copper washers that fit perfectly for a crush washer. I'm not sure what the original application was.
When drilling it out, I put some assembly grease in there to catch any shavings. After all was said and done, I also poured some oil through the engine as well to wash out anything. 19mm socket fits on the 1/2" bolt head perfectly.