"How do I test/fix the compression?"
Remove all plugs and coil wire. Open throttle all the way.
Screw in the compression tester in first cylinder. Crank over engine 3 to 4 times and write down results.
Proceed to next cylinder and repeat.
There should be no more than 10% variance between each cylinder.
Now, squirt approx. 1 tablespoon of oil in each cylinder and repeat test.
If compression increases, it indicates a problem with rings; if no difference, it's a problem with valves.
I would think it would have to be a lot of variance in order to have a big power loss, though.