Water will not harm an engine. May add some mass depending on the mixture and speed things up. However when lightening is present, that is not a good thing as static electricity is in the air. Most any old CL stunt flier that ever used a handle with an adjustment cable on the back side of the handle can attest to the shocks that happen when doing maneuvers especially in the top of the circle. Actually burned my hand a couple times.
There are numerous counts of golfers getting it on the backstroke on the golf course and graphite rods have suffered a few fishermen lightening strikes in boats. In my 41 years of aviation, I never got used to St. Elmos Fire -- build-up of static electricity on nose of airplane -- when in ice crystals and/or Sirius clouds near thunderstorm areas, sometimes up to 40 feet or so of blue flame around the airplane.
So simple rain is one thing but when TRW stuff is the event, please shut it down and take shelter so you can fly another day. Don't play with static electricity.

BTW, if you crunch an engine, and it's really dirty, wash it off with a water hose before moving anything. Get all that dirt off. Then take the carb off, the back plate off, the glow plug out and wash in wood alcohol or rubbing alcohol, then oil it up for good as new.