Have been using
FROST POR15 paints for ages - not the cheapest in the world but really works.
One reason is that if you only need to 'touch up' a very small or large area - just clean with brake fluid and apply with a cotton bud or rsoft paint brush - and end up with an almost perfect match without gloss and brush marks.
Done my cylinder heads and footrest hangers amoungst other small areas and you can't see the join.
Also the sump parts and exhaust front pipes in matt black - lasts for ages.
Bit pricy when I bought it - £15 for 1/2 litre - but well worth it. Never tried the gloss but well worth visiting the website -
www.frost.co.uk/Paints/Engine.