According to the scans from Clive, the Mk1A has JAP 55 engine, this is a twin cylinder and dates from 1940's, the Mk1B had a Petter PA2 which I am sure was introduced in early 1950's. This leaves the Mk1, which appears to be an air cooled flat twin, but not entirely sure what make. I am sure it is not a Petter PU8 as the spark plugs are in wrong position, this was a wartime engine (there may have been an earlier version?), other flat twins used my military, were Norman, Bradford, Douglas and Coventry Victor, but not sure if it is any of them. The owner of the restored one in blue paint might know.