i think all minis' had 4 cylinder motors ranging between 850 - 1270 cc, depending on variant. Only after 1984 only 2 engine variants were sold in RHD countries.
in uk there are mini austin clubs if you connect with them they can guide you best way to reuild the engine and restore such a classic to oem specs..
japanese motor surgical implant will completely destroy such classic. @msabbasi saheb being a classic car owner may guide you for real meaning of classic.