umm i belive that they do make 185 50 r13 for minis and they used to do a 195 but they rubbed on the inner archs at the rear with rubber cone suspension on the minis.
i know you can almost easily get 185 60 r13's but for that real low profile look, the 50 serise would look good.toyo tyres make these in 185/60/13 and also 205/60/13