Try the stripper that's good for fiberglass - it shouldn't bother the ABS parts. Test it on a small area first, just to be sure. I have stripped fiberglass and urethane using the aircraft-type stripper, both worked very well and did not damage the base materials at all. If you scrape vintage plastic, it's gonna show when it's clean.
Last swap meet shroud (69 SB) that I found, really grungy, cleaned up to the point it looks NOS by using a soft brush, dish washing detergent, followed by a liberal coat of Detailer's Advantage - after the part dried off, it looks better than the NOS one I have in inventory. $5.00 shroud, by the way -