There's many types of icing. I don't know if structural icing is modeled (getting ice on your wings for example, that would kill your lift), but i think i can make a good guess about what happened to your aircraft.
If you just fly close or through the edges of a cloud you'll get pitot icing which drives your instruments crazy. Turn on the pitot heater to fix this (i have it mapped to H). That being said, i don't know if all aircraft have a pitot heater.
What you describe though indicates a clear loss of power. This could be due to carburetor icing. I had a similar but more critical failure a few days ago, i was flying a G.50 which doesn't have a carb heater and decided to fly through a cloud to see what will happen. Well, the engine choked and seized completely because the carb was completely blocked with ice.
The Spit also has an engine with a carburetor and it doesn't have carb heat controls so what you experienced looks very similar, you just happened to exit the cloud before your carb completely iced up.
Any kind of humidity in the atmosphere is a carb icing danger, even on hot days, so it's best to limit flying near or inside clouds if your aircraft doesn't come equipped with a carb heater.
The exception to this is if you are running an injected engine, which is another advantage for the 109/110.
Hmm, now that i think of it, that's what i'm going to do online if i can't shake somebody, just dive into a cloud and wait until his engine stops
