Are there undecorated shells available for these engines?
no
91% isopropyl alcohol will safely remove the paint, with a little bit of scrubbing with a soft tooth brush.
put the shell in a container filled with the alcohol, seal, let stand 10 minutes, remove scrub, rinse and DRY (drying is very important), put back in the alcohol if required, repeat. Different paint schemes take different amounts of time to remove. Adjust your times accordingly.