Jim,
I bought some 17" wheels for a Bonneville Comet project. I think I checked that I could get new narrow (relatively) tires for them from Bridgestone before I got them, but that is about it. They were 250 Ninja tires. There is at least on racing org that runs a class for 250 Ninja's, so there should be some good info available on the tires that they are using.
The reason I went with mag wheels is that many land racing orgs. like DLRA in Australia have mandated mag wheels for safety reasons. Mag wheels are not required here, but are allowed and the inspectors like them because they are tubeless and avoid the rapid heating between tube and tire. Conditions here on the salt are so bad now that the lighter unsprung weight is also an advantage.
The rules for "M" or modified class is that the seat has to be as high as the top of the wheel rims. Thus, the smaller the wheels the smaller the bike. In the A or altered class, the seat can be as low as you wish, but I thought I might try for M. Considering how quickly I am producing work, this project will be done about 4 years after my demise.
For the 17" spoked Vincent wheel the only thing I would worry about is that the flanges have enough angle so the spokes don't bash each other. Peter Barker has pointed this out in other posts, but the flange needs a little more bend when you get down to these small diameter wheels because the spokes are so short, which makes the path to the center of the rim a little tighter.
David