Welcome to MTF! Lots of info here and great members that will help as the can, enjoy the site! Sorry, first time I've ever heard of a mini-BMC and I see that Kubota Ronin has provided some information on it.
The motor is a BMC A-Series, 948cc diesel which was produced for only 3 years1965-68.
The A-Series engine line up starts around year 1950 @848cc and runs thru to the year 2000 @1800cc
The casting number on the head is 12A735 Which is located on the back end top of the head