[Sidecar Bob]

By "head cover" you mean the valve cover, right?
Unless something is very wrong there should not be coolant in the valve cover (not to mention that the rubber seals should keep anything that is in there in). Your first step should be making absolutely sure where the leak is.

 

[Sidecar Bob]

I can see them calling head covers (they are often called rocker covers too) but I wanted to make sure we were talking about the same thing.

The water pump is at the rear of the engine. The thermostat housing is above the engine between the cylinders with a pipe running each way to joints on each cylinder head.\

As Randall said, check check around the thermostat housing. If there is a leak at either end of the pipe, the thermostat housing or even where the rad hose attaches to the thermostat housing it could easily run around the cover and drip off at any point, looking like it is leaking from the cover seal.

 

[Sidecar Bob]

The power loss & bogging is probably unrelated. Unless the coolant was getting onto the spark plug wire and the wire's insulation has developed tiny cracks or one of the boots on the spark plug cap are leaky.

 

[Sidecar Bob]

An o-ring on either end of each of the pipes and one between each joint (elbow) and the head it mates to, another between the top & bottom of the thermostat housing and one on that seals the temperature gauge sender to the housing. That makes a total of 8.

Looking closer at those pics, it looks like the leak might be between the joint and the head but it could be leaking higher up and running down the back of the joint and around the bottom of it. You need to look up underneath with a flashlight to make sure it isn't coming from somewhere else. You don't want to replace all of those o-rings and then find out the problem was a loose rad hose clamp.....