Rocket Exhaust for ME-163 Komet

About the only thig I can thik of is to use acrylic rod covered with cotton to simulate a smoke trail. If you are a bit more adventurous, you could use an acylic gel over the rod and shape the smoke trail to your liking. That way, just means more painting though.

There was an article in FSM where a guy did just that - used shredded cotton balls wrapped around a wire armature to lift an 1/72 Me-163 off the ground at the moment of takeoff - looked pretty good - I'd think the finer the cotton balls, the more realistic the smoke. Have a look in the FSM index for the month/year and have a look at the article - it would be a good source for tips on painting the different colors of the exhaust.
Don't forget the steam vent as well.
Cheers, Lee Tree