I know I've heard of going the other way, but I can't recall anyone putting the 180 head on a 200. Kuranosuke is the motor rebuild expert, maybe he'll stop in and comment on this. I believe the 200 head has a bit bigger chamber on it, so going to the 180 head would tend to up compression. More power, but more picky on the gas it will want to run. Might have to time it a little less advanced too.
If you haven't gotten yours apart, it is probably just the gasket that is leaking. It is possible the head cracked or warped enough to be unfixable, but usually it is just the gasket. That said, I just finished head work on a truck that did have a cracked head.

If your budget allows, take your current head in to a machine shop and have them pressure check it. It may be good enough and you could avoid the worry. It would be worth getting the valves/guides done, but it depends on if it is worth the price.