I don't know if there are many 18" tyre options available unlike 17" so you could be looking at a overall size change.
Measure the width between the swingarm at the widest point of your tyre to give you some idea of the max you can get into there.
A reasonable start point maybe (if you want spoked wheels) would be to go with an F1 rear wheel and have a wider rim laced to it or even a K7 17inch rim if suitable.
Unless you have a reasonable skill for this sort of mod you may not get too far without some fairly involved changes.
Saw in a recent USA retro bike mag a CB650 with a modified rear comstar to make it much wider, think he had the rim machined off to leave just the centre, then a bespoke wider rim welded to the remainder.
