RE: Welcome to Club SAITO !
Try oven cleaner (the foaming type) but don't leave it on for more than a minute or so (will chew up the Alumimium) looks like it will need several applications. Scrub with an old tooth brush and wash off with water. Don't forget to strip the engine first. Works on most engines.
I have found this sort of residue is due to to much castor and not enough lubricating oil in the mix causing overheating over time