Is there ANY chance that the coolant froze? ( either over the winter or after the timing gear project...be honest!). Sure sounds to me like a cracked block, cracked head, or blown head gasket. Hopefully (probably) the latter of the three.

If any of those are the case, then the spark plug in the offending cylinder should be clean (if it is hitting at all..if not, it could still be dirty). Try running it from cold for just a couple minutes...don't let it warm up totally. Then shut 'er down, and pull the plugs to see which is wet. Wet being wet with water. If you pull a plug, and its full of water...well... cars don't run so good on water. It just doesn't burn very well.

If its really bad, you can pull the plugs and insert a clean white piece of paper into the spark plug holes till it hits the pistons. If it comes out wet, you know you have a problem. Might be best to yank the coil wire and turn it over a few times before you do that....(plugs in, air intake plugged so you get max vaccuum to suck water in).

And of course, a compression check would certainly be in order I would think.

I can't imagine that the timing gear issue would have/could have caused this though. Heck, it hasn't run all winter. Coulda had a lot of condensation in there. Filling the coolant bottle won't necessarily fill the engine, but a quick start would make the water pump fill it, and hence..drain the bottle. You might be fine just to change the plugs, and go...go...go...in your Vol-vo.

Just some shade-tree thoughts. Good luck, and keep us posted!

-Matt