[cleanspeed1]

Well, it depends on how far you want to go. If it was me, I would put together some money and schedule a rolling road session with a specialist that has wide band O2 sensor capability and optimize the carb and ignition. Those old engines did at least have a decent combustion chamber. Also, do a compression and leakdown test on the engine to make sure that it is in proper internal health ( do that first, my head was running in reverse when I started writing this ).
Going further in, and this is along the lines of what people have done with the Geo Metro, do a cam swap. I know with your pushrod motor it's a little more involved, but the payoff is great, especially if you are not racing the little bugger. I would talk to Kent or Piper and ask about a maximum torque/fuel economy grind that boosts the torque in the idle to 3000 rpm range, and short shift that thing like a diesel. I am not sure if anything can be done with the gearing in the Minis ( you have the one with the integral gearbox, right? ). Look at the differences between a standard Geo Metro and the XFi version and pattern your car along that line.
The carb does not have a fuel cut feature, unfortunately, but the good news is is that your car is so small and light that with a few changes you'll be way better mpg wise than right now. I thought those small engine Minis were in the 40s or better fuel economy wise ( US ).