If you want a wire spoke wheel, get the rim sizes of your choice and have it laced up to the stock hubs. You can find used rims cheap on eBay. Then all you need are the correct spokes.
You can go with an 18 on the rear, but you need to be careful with tire choice or it will interfere with the swingarm cross bar. However, you can notch the cross bar for clearance. Here's an 18 laced to the stock rear hub:

A 17 might be a better choice, because then your tire options are huge. Sportbikes, tourers and motards use 17s. It will be harder to find a cheap 17" wire spoke rim, though, because all the motard guys want them. They can be expensive new. But if you go that way, you'll be looking for front rims and tires, because the rears would be too wide.
Things are less complicated on the front, assuming you're not using a fender. A 21 will fit fine, but check tire options first. I haven't needed to look for 21" street tires (rather than knobbies). You really want to stick with the stock front hub, otherwise you open a can of worms with the rotor and caliper.