My grille wasn't as intricate, but it looked like green scum over years of sitting. I knew I could never find a replacement, so I decided to clean and polish it myself.
I built a frame from old wood and then lined it with plastic sheeting to make a soaking tank. Then I mixed up simple dishwashing soap and hot water and dropped it in. Scrubbed it with a soft bristle brush as it soaked and almost everything came off.
Afterwards I hosed it off with plain water and used a toothbrush and liquid chrome polish to scrub and clean a bit more. When I was done I hand polished the surface areas and it came out great. I have some pits in my chrome, but they are OK by me.
I spent a day on the project, maybe 6 hrs. total, a couple more carefully straightening some bent grille bars, and well worth it to me.