Starts better warm than cold.  Actually this was the case with my inverted TT 91 till it was well broken in.  I mean like a few gallons, not a few tanks.  This engine like Saito takes a long break in.  The fact that it has collected oil and dirt over the years making it look old doesn't mean it is actually broken in.

Give it some time.  Also consider going to much lower oil content.  Four strokes really don't need 18% oil because some of it is retained in the crankcase. Inverted four strokes run better with about 14-15% oil.