HD-10,-14,-20 all had Detroit's(GM). Early 50's GM bought Euclid and AC decided to produce their own engines rather than buy engines from competitor. Enter BUDA exit GM. Early HD-6s had BUDA and Allis Chalmer markings on the engines. All HD-6s had the 344 cu in 4 cylinder engines model HD344. Later called 6000 and 7000 (turbo). I have heard stories on the net that BUDA is back in business.
I have seen HD-6s converted to Detroit 3-53 power should be an easy swap with good power and parts availability.