Tdr front wheel conundrum

[acridjacket2t]

Hey all..
Well, finally taken delivery of a gorgeous full gianelli system. Fitted and tested. Wow, what a transformation. Very happy.
My next job is that of the front wheel, or to be precise, either fitting a suitable 17 inch front wheel from a donor bike (tzr 125 90 on?)or maybe a 90/90 18 tyre. Why the wide 100 as standard? ?
Any thoughts much appreciated. (Still can't figure how to upload photos)