Hi Paddy,
You have two options: gather all the bits and build from scratch or easier one get the kit. They come up now and then on nutz.com.
If you building from scratch you'll need: sc (m45 cooperS, more powerful m62 from mercs), 1.6 sc to engine mounting brackets, belt tensioner, throttle body, bypass valve, vac hoses, outlet pipe, various blanking plates, management (standalone or piggybacks). I've done a lot of research and went with the kit after all. All of parts can be easily made if you've an access to workshop and handy with the spanners and know how to weld.
Have a look here:
http://www.mx5nutz.com/forum/index.php?showtopic=42329.
I've 1.8 Jackson Racing m45 kit, dual tb setup, Ms3, stock pulleys. Still in progress..