A vacuum leak or misfiring could cause a low idle but what I would have a concern with would be this “oil was extremely low” part of your statement. Just how low is “extremely”?
You should be aware that cylinders and piston rings are lubricated by splash oil and continued operation with little splash is going to damage the piston rings. This in turn means engine compression problems which could cause a misfire if low enough.
Just based on what you stated about the oil being extremely low my suggestion would be to run a compression test and make 100% sure that the engine is even worth throwing parts at.
Any compression problem that exists should be diagnosed further to make sure that it’s not a tight valve lash problem that has surfaced and the rain was actually a non-factor.