Bob Severance and I saved someone's annual fly in that was being rained out by flying in the heavy rain one morning. Touch and goes in my intrepid Kougar were. . interesting! After we put on our demonstration of how easy it was, others joined us and the fly in became a success. A pop up was raised on the flight line and we began to have some serious fun. By the afternoon the rain had left.
Wear a poncho and put your transmitter under that and go forth and aviate, I carry 2 as emergency items in my trailer and car.
The comments about engine runs might be of note if it is a toad strangler of a downpour and your carb is facing up and out in the open, but our stuff was all 90 degree installs.