found a cheap set near me and wondering if they would fit but I've heard the centre bore is the wrong size, is this true? and is it a quick fix?

As far as I'm aware the Mazda MX5s have a center bore of 64.1, as VW is 57.1 the will fit. But you may want some spigot ring 64.1 - 57-1 to center the wheel when putting them on, only a couple of quid to buy these.

Hope this helps.

Pete