Spray the cleaner on a toothbrush and then clean the engine (with the model nose down or held so that the drips off the engine don't fall onto or into the model). Wipe up with a paper towel or, better still, a cotton rag.

I usually use 91% isopropyl alcohol. If you use a carb, crud or brake cleaner be very careful of the Monokote or whatever covering. I'm not sure but some of those may attack glues and epoxy, too.