Flyin Miata will supply you with pretty much everything you could ever want for an MX-5.
Forced induction is the cheapest when it comes to how much power you get for your money, and you'll be able to get a decent amount out of the engine before you have to start strengthening the internals.
Flyin Miata supply several turbos for Mk1 MX-5's and you can get superchargers from Jackson Racing, Performance 5 or use an Eaton M45 from a mini. The mini one is probably the cheapest, takes a bit of work though.
The cylinder head has room for improvement aswell.
Take a look at the UK owners club forum:
PM me if you have any questions