Measure the length from bottom clamp to axle if it's shorter than stock you need to seriously question what you are doing. None of this will be any improvement in handling only in braking, damping and wheel tire choices.
You may want to familiarize yourself with motorcycle geometry and do some measuring to understand current rake and trail, not just the numbers from specs but how it all is measured and calculated, this will be the determining factor in the end result... just swapping parts could put you in serious danger when the bars are violently trying to make you let go and crash.
All that said, you can buy the stem and all balls kit from Devin: I would just get the parts needed to do the swap then really think long about spending dollars for cool parts like spoke hubs and Excel wheels buy used eBay wheels with good tires for testing.
Stem and parts on site:
https://cognitomoto.com/collections/fork-conversion-parts/products/yamaha-r6-r1-fork-on-honda-cb650-frame-conversion-stem
As always... disclaimer in sig <-------<<<<