If the sign shop could make you some good red flames, just buy a can of that new "rustoleum universal" in orange and sorta dust the tips and edges of the red vinyl decal before you apply it to the cart. That way you'd have a fading effect against the yellow background of the cart.
Or buy some 1/8" masking tape(available at just about any auto paint dealer, or on eBay), and tape off your own flames...being 1/8 of an inch, its pretty easy to curve, then fill in all the blanks with regular 1" & 2" masking tape and/or newspaper. shoot the whole thing with red, then dust your tips with one color, your edges with another. That way you wont have the lip that the vinyl decal will leave.