Not gonna happen. The only way I can see to remove the decal is to apply masking tape and try to "rip" it off... but you'll probably wind up ripping the tissue paper. Since you say acrylic paint was used, then the decal applied on top of that, you might try soaking the decal with some water applied with a brush... maybe that would soften the decal's glue and allow you to scrape or lift it off somehow? I suggest just painting over it.
I agree. Tamiya tape (comes in 1/4 inch and 1/2 inch widths) is the best product to do color separation lines. Razor-sharp lines, sticks well so no bleed, yet pulls up easily and won't take paint with it. It's a little pricey, but it's the best tape I have found so far for masking off bodies.
Another way to make mufflers... cut some large aluminum tubing to length, then carefully flatten the tube to oval shape by placing a small piece of wood on the tubing and then gently tapping with a small hammer. Don't go too far, you want to get an oval cross-section, but don't want to totally flatten the tubing. Muffler ends made of styrene sheet, drilled to accept solder pipes. Smaller diameter aluminum tube cut to short lengths and slipped over the ends of the solder pipes to create the open exhaust tips. I don't see the advantage of making the pipes by using styrene tubing with brass rod inserted into it when solder alone can be bent easily and will hold its shape just fine... yet is soft/bendable enough to allow you to fine tune the fit easily.