without marring the plastic?
I haven't found an easy way. The ink used to print the lettering is tougher than the paint used on the shell. I have found it easier to strip them completely , repaint and decal. Anything done just to remove the lettering usually ends up leaving a ghost behind.
some body told me to use comet cleaner and a soft wood stick to rub the lettering. I have used this method on may be 8 engine and cars and have been happy with the results so far.The only problem I found is that it leave a slight shinny spot on bare plastic. Only my results you need to test for yourself.
Sorry forgot to mention to make a wet paste out of the comet then rub the lettering a bit then rinse check and repeat if needed.