I'd try and keep it original
as for colour, the Hetchins I came across as mainly original in the early to mid 70's were usually sombre dark metallic colours.....black, green, purple and the unusual ones which pushed the boat out were a Bianchi type blue (less greenish than bianchi).
Deep purple or a darkish metallic green with tastefulk chrome always look dignified
The bike does have evidence of some chrome under the hand painted black. My grandad had several Hetchins in his time, one I know was all chrome and the other was a lovely metallic/pearl blue. Both bikes were made to order for him in the late 1940s - and both he sold in the early 60s. He tracked the blue one down in the late 80s and had it refurbished - frustratingly he sold it again about 10 years later - a wheeler dealer until the end! The bike I have was built for one of his brothers and came in to his possession.
Colour wise I have seen one in a powder blue colour I think would look good.