I'd use citrus based degreased to begin. Once the grunt of the grease is gone, I'd go acetone, one acetone soaked towel to apply, one dry and clean one to wipe it out immediatly (it you leave it it will attack the fiberglass).
Then only I'd start sanding/scrapping.
Instead of acetone, you can use lacquer thinner. It evaporates a little slower than acetone and so it's a little more economical to use. However, evaporating slower, it may attack the fiberglass a little more.
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My 68 had significant oil leaked from the valve covers and who knows where else. Over the ages the oil, and in some places engine heat, had turned the oil into an almost varnish like substance; i.e. a substance permeated with sand...there's a lot of sand in the air here. I scraped it off with a putty knife and a rotary wire brush powered by an electric drill. ...then next countless swabbing with at first acetone and then later lacquer thinner. The upside was that once I got all this stuff off and cleaned, I had a frame that was as pristine as the day it came from the factory. Breathing acetone and lacquer fumes, particularly while drinking wine, stresses your liver. I stopped drinking for about 9 months. To do it over, I'd definitely wear a face mask with charcoal filtration.