I have no idea what leaning the bike over and spilling gas would do, but I suppose it wouldn't hurt. I suppose if nothing else it would assure that you are getting gas to the carburetor.
Getting wet can often lead to ignition problems but if you have good spark then you can pretty much rule that out.
How is the compression? Does it still feel as good as it did a couple of weeks ago?
If you are getting spark, and gas to the carburetor, and the compression is good, then about the only thing left is the intake & exhaust. Just how hard was this rain it was left out in? Any possibility that the exhaust filled up with water? Out here in California we never get a rainstrom that could do that unless the bike was parked directly under the downspout from the roof, but a rainstorm in Maine might be a different story.
If water filled the expansion pipe then it might not be allowing the exhaust to flow properly. If water filled to the point that it ran into the engine then all sorts of problems could result.
If you are not sure then I would recommend taking the pipe off and dumping it out. If it turns out to be dry then you don't have any worries. On the other hand, if you get the pipe off and pour out a significant amount of water then you should suspect that some got into the crankcase as well.
The standard method of clearing a water ingested two stroke is to shut the gas off, pull the plug then turn the bike upside down to get the water to drain out the spark plug hole (note that the piston must be at the bottom of the stroke).
Rod