The standard Lotus engine (at the time) was the 2 litre Talbot Sunbeam derivative, which had the usuall Chapman treatment, i.e twincam head and twin 40’s (sidedraught carbs), this engine had 2 further incarnations, as a turbo, and a dry sump turbo (later engines were also charge cooled)

Wrong way round. The Talbot Sunbeam Lotus had the standard Lotus engine.
Originally derived from, or tested with depending on where you read it, the cast iron slant four from the Victor FD. The first few were built with Vauxhall blocks but quickly went to aluminium Lotus blocks.