Gary,
'IF' i were to go with an 18 front... Would buying 36 18 inch spokes work with the original front hub?
Yup
https://www.mikesxs.net/yamaha-xs650-stainless-steel-spoke-set-18-front-36-spoke.html
I spoked my 18" front with those.
I'd post a pic but already posted it a zillion times.