Are you talking about the old mini Cooper with the 'A' series engine? As they were originally only 803cc and the last ones made were 1300 cc +- I think you would be pushing it a bit to bore it out anymore, apart from the thinness of the walls you won't have much head gasket between the bores. Put a turbo on it.
The MG Metro turbo A+ unit would drop in, but I wouldn't want to bet on being able to find space in a Mini for the induction/exhaust/turbo arrangement.
Twin SUs, a tubular manifold, decent exhaust, a warm cam and a gasflowed head are probably worth more than 40cc of capacity.
