1999 Buick Century Diagnostic code cylander 2 miss fire

What did the plug look like that you pulled out?

Intakes are common on these

It needs to be diagnosed by a shop that normally does this work.

I doubt a coil problem, unless there is corrosion on the coil for that cylinder. I would expect two cyls misfiring other wise. A spark tester can confirm this

The fuel injector should be checked to verify it is working.

HAve you been losing coolant at all?

The cylinder can be compression tested in the event it is the head gasket.

To help check for an intake leak you can:

The cooling system can be placed under pressure overnight and the oil drain plug slowly released to see if any coolant comes out first as it will settle to the bottom.

Or

have the oil drained out overnight with the drain plug out and see if any coolant is caught out

The coolant flush may have put the gasket over the edge. I wouldn't be surprised if those last two tests yeild coolant to confirm it.

Suppose I should have specified intake GASKET leak.

I really can't say I've found these to be a vacuum leak kind of thing on this engine, But great point about checking for intake leaks regardless.