Crazy thought, but Triumph TR 4 cylinder, same character as a BN1 engine, torque, low revving, about the same power, Lucas electrics. Much easier to find than a BN1-BN2 mill, cheaper parts, lots of go fast bits. I do thi k the carbs are on the wrong side, so that is not so good. The Volvo engine has been used in a lot of British cars, don't know about big Healeys though.