If you are thinking about drilling and tapping your oil pan, at least remove it first. Tapping Aluminum isn't all that simple and you want as stable and level a surface as possibly. Also, remember you need your new bolt to fit up as close to flush as possible (as in make sure you are tapping perpendicular to the plane of the oil pan), or you gets yourself a nice mild drip. Personally, you could probably find a replacement pan that would work out cheaper than the possible aggravation of fiddling with it and not tapping it right. Aluminum isn't too forgiving. Plus there's the game of finding a nice flanged bolt to use for your new oil pan plug.
Helicoiling would probably work a bit better if you are intent on not buying a replacement oil pan. The person doing it would drill out your hole to a larger diamater and then insert the helicoil thread and then get your bolt in there so you are left using the same bolt as before with a diameter of hole which is also the same.