We wanted an 18" rear wheel, and came up with a solution.... I took the wheel apart and sold the 16" rim, then found an inexpensive whole-wheel from a TX750, which, after some research, I found used the same 40-spoke rim.
Here is the cleaned hub from the XS650, compared to the as-found TX wheel:
Test assembly prior to decision to powder coat hubs and rims.....
And this is when we ran into "supply" problems... we wanted to re-spoke with stainless steel spokes, and there wasn't a set for the rear wheel to be had....front wheel spoke set WAS available: