Dave, I think originally the Clippers were Perkadyne (Perkins) powered from Ansair. The Deutz and Detroit were popular retrofits, both popular and successful engines in the day. The rear axle was Kirstall.
Clippers were built between 1950 and 60, a time that Pommie imports had a major advantage over any non-Commonwealth country.
GM Detroit Diesels were not available as loose motors before the mid 50s. The sale of loose engines was forced on The General by anti trust laws, introduced by the US Fedral Government.
Fixable would supply all brackets and mounts for owners to convert the original straight eight Buick (?) to GM. 4-71was a popular conversation in the States.
There is an active Clipper club in Australia.
Clippers in Australia | Flxible Clipper Club of Australia