Alex, you don't list your location which makes things a bit difficult. There are a few members on here that manufacture frames.
Suzuki Swift GTi's seem to fit Roundies quite well as do Diahatsu CB70 3cyl. Example of a CB70
http://www.ausmini.com/forums/viewtopic.php?p=733067#733067
The most popular for a Clubman is a 4EFTE from a Starlet Turbo.
I had an interesting thought yesterday as my Son just bought a Honda City (new one not the old fugly things) which has an L15 i-VTEC engine in it. It is a very neat little package. He'll be here later today so I'm going to take photo's and get the Tape Measure out. It's no Fire Breather 88kw at 6600rpm and torque is 145Nm at 4800rpm but would make a nice smooth motor.
Dicko