Last night I started playing with some parts, looking at a 17" allow wheel conversion. The aims are to reduce weight & rotational inertia, improve the true-ness of the wheels, and allow the use of low-profile modern rubber. I'm also trying to keep this mod needing only off-the-shelf parts and basic hand tools (like a hacksaw and a file) for non-precise fettling. So I'm swapping bearings to avoid making a new axle, buying stainless seamless tubing the right size for the inter-bearing spacer and just trimming it to length. Proposed wheels at from a KTM Duke/RC in 125/200/390 capacity. The early models have the same size disc too. These are pretty common and cheap in the UK, cost me £80 for a pair complete with discs, tyres, and sprocket.
So far the new from is 0.7kg lighter but also has the mass much closer to the hub, so should really improve matters.