I thought the current MX5 had a relative of the Ford Duratec?, the 2.0 duretec is easily uprated to around 200bhp. Its a newer design and better in standard for than Zetec...
Practical Performance Car did an MX5 guide a while back. From memory they said the Mazda engines didn't respond well to 'normal' tuning (porting, etc) but did like the old forced induction. Believe that's a well-trod path so may be less hastle.