There's an interesting reference to an Yimkin powered Mallock in this extract from a 1961 750 Racing Bulletin - U2 News - written by Arthur Mallock himself. The car was owned and built by John Heseltine and raced as the Vanford U2. It had several successes and finished in the first ten of the 1172 Championship (with over one hundred cars competing) in 1961-2 and 3. For the 1964 season they decided to make an engine change and a Vauxhall Viva engine was installed and the car then raced as a the Vivan U2.
No news so far on what happened to John's Mallock or the Yimkin engine from 1961 but it's a great reminder that even if the cars are no longer with us - it's likely that the engines will be 'somewhere in something'.