Before you get too far along, where on the motor is this bolt and leak? If near, like in just above, the oil filter, I had a similar issue that turned out to be a clogged drain for water, rain, fuel or whatever collects between the motor and transmission. It looked like a rusted off bolt, but it was actually part of the drain tube that was stopped up from rust and small debris over time.
I cleaned that out really well and no more water leak for me. What I thought was a coolant leak was simply a drain hole that was not draining well so when you drove it, the water blew all over the undercarriage and dripped all over my driveway, making me think I had a coolant leak.
We have since had a very heavy rain more than once, and once the roads were dry, so was my engine and driveway, so it was just rainwater that couldn't find its way out of the drain quickly enough.