I have used a convertible top cleaner that I got at the auto parts store. It was simply called "Convertible Top Cleaner" This was 7-8 years ago, but it should be available. Mild, but works well...at least on my black top. With your ivory top, some Westley's Bleech-white tire cleaner might work very well. I've used it on vinyl's and white walls, but you'll need to test a small area first to avoid the unthinkable!!
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Jeff 
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