Very popular in the UK is using Honda 1.8 Vtec engines or the 2L Vauxhal twin cam (whats in the Holden Vectra). Check out Mini World http://www.miniworld.co.ukfor more info.
One guy is getting 200bhp from a 2L motor (no turbo) in a mini weighing 600kg!!
The hassle with what you have, if it's an A serise engine, is you are at the very limit of what the block can take on over bore (1430cc is usualy the safe upper limit fwik) so adding forced induction may well reduce the life of the engine and give you more grief than its worth. Turboing a 1380cc A serise is a common thing, and Mini World has had many articales in it over the years.
Better still, just put a 4A-GZE in the front mind you, it may not be road legal then... worth checking first!