keep in mind 50-year old plastics are brittle, and can crack and break easily. more easily that you would think, so handle it with gentle care. a newer repop still has some flexure. the older stuff does not.
if that brown is rusty metal residue, you'll want to get that off first, as the grit may scratch the plastic as you clean it. for this, you can soak it in straight (5% off-the-shelf) kitchen vinegar. or something stronger, if you want. I often use an old credit card as a scraper. In this scenario, after soaking, you can scrap the metal off, and resoak until clean.
next is your basic soap and water. you want to remove as much or all of any dirt as you can. I like using liquid dish soap and water, mixed 50/50. if there are crevices, an old tooth brush works great. I even have an old sonic toothbrush I use on occasion.
finally, treat the fine scratches. that is a textured (ribbed for her pleasure, in this case) lens, so it may not show any fine scratches. but a side marker light, instrument cluster cover, or tail light lens may. fine scratches can cause a haze. for over 20 years, I've been using an aircraft (most have plastic windows) acrylic plastic cleaning product that comes in three bottles. similar to this product from Amazon. it removes the scratches that can be removed quite well. just takes patience and a little elbow grease.
