I used a CR80 Hub, laced to a 17" wheel. Used the CR80 disk also. Along with MB5 forks and caliper.
Off the top of my head, without going down to the shop and looking at it. Pretty sure the hub fit right between the fork legs with little or no spacer work. The biggest job was milling the inside of the caliper to clear the CR80 hub/spokes. The CR80 is a fairly wide hub, so the spokes are farther out than the spokes on the cast MB5 wheels...so the spokes interfered with the caliper. Had to shave a few MM off the inside of the caliper so the spokes could rotate without touching it. Other than that it was a fairly "bolt-on" affair.
It's worked perfectly since I did the mod about three years ago.
Milling the caliper isnt a problem since I have a small mill. I can see why you went with 17" but that isnt gonna do me any good if I cant get tires. I'm also looking into a set of Kawasaki AR 50/80 wheels to use for this project. They would look sweet and would prolly be cheaper than a spoked setup plus they are pretty lite I put the word out to the VJMC and a few guys that race 50's over in the UK that I am looking for some so I'll see what happens.
There still are 17" 50cc race tires out there but you have to get them from Europe.
I think the CR80 front rim...at least the year I used is 17" anyway. So if you don't use a cast wheel you'd have to spoke something up which can add to the expense.
Easier to get 18" 50cc race tires.