That bolt holds the oil pump housing tight against the o-ring that seals it to the engine block. As you found not tight equal big leak! It should be tightened to 9 foot lbs which is nice and snug on the hand built in torquer scale. Just like Windows you'll need to either reinstall it with a new cover or do it like linux and apply a bitchin' patch. Just like in the computer engineering world you'll find the the bitchin' patch cheaper!

To go the patchable route go to a tool store and buy a Helicoil thread repair kit for your Toyota 2.0. The size you'll need is M8x1.25. When using the tap in the kit put grease on it to catch any metal shavings. Use a wet/dry vac after to get it all.

You need to get a bolt of the right lenght. Too short and it might strip, too long and it will hit the chain tentioner. So go to a hardware store and buy M8x1.25 bolts of different lengths longer and shorter that you think are close to the size you need. If none of them are a perfect fit take one thats a little longer and use a grinder to make a custom bolt. Be sure to dress the threads up so they smoothly thread into the cover. You can thread a sacrificial nut on before cutting and this will do the trick when you thread it off. Good luck and may the force be with you.