That looks like a solid 18" wheel blank, machined to be the same appearance as the 1988-2007 rear wheels. Looks like it's well-done, to me. Our wheels are unique to the bike, they were never used on any other bike besides the Gen. 1. What did you pay for it?
The wheel inside the rims should be something like 5-1/2" to 6" for radial tires. A 170 to 190 should fit, but either a 170 or a 180 would be best on that size 18" rim. A 190 might rub the left side of the swingarm, and require you to do the "washer swap," where the left washer is placed inside the final drive case and against the wheel, providing a bit more room for the tire.