Sylva recommend 14x6 wheels for the Mojo.  The 2003 Mojo race car used 195/60 rear tyres and 185/55 fronts to give a good balance of grip front to rear.  Customer cars are frequently using 185/55 or 185/60 tyres all round.  It is also feasible to use 15x6 or 15"x6.5" rims with slightly lower profile tyres to compensate- the latest Sylva demo car uses these, with 195/50 tyres squeezed under the arches at both the front and rear.  15x6.5  are more widely available than 15x6 (A set of Ford Focus wheels may well be the easiest way to get this size of rim?).

I initially ended up going with a set of 14" wheels with 185/55 tyres, as that is what came up on Ebay (for 150).  However, when I swapped the tyres the first time I went up to 185/60 R14 Yokohama A539s - see article in the mods section.

In 2009 when another tyre change was due, I noticed that 15" tyres were now significantly cheaper than 14" tyres, and offer a much wider choice of, so bear this in mind when making the decision.  I ended up buying a new 15" wheel and tyre package for not much more than the cost of a set of decent 14" tyres.  Just bear in mind that larger wheels and tyres will be heavier, I chose my wheels and tyres carefully to minimise the weight gain - see further article in the mods section.

The alloys themselves should be 'standard Ford 4 stud fitment', i.e. 4 stud fixing, 108PCD, 35-38 offset & maximum  6.5J width.  Certain wheels from Ka, Fiesta, Escort & Focus should fit- these often come up on Ebay.