Yes, as you suggested - wash out the degreaser after gentle agitation with a fine brass brush, then blowtorch. Exhaust port nice and clean.
It still stumbles a bit like a spark or over-rich mixture issue (sounds “fat” if you know carbureted cars and the exhaust note changing with richness/leanness), but will actually rev hard now.
Your advice has got it going so much better though - thank you!