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Why doesn’t anyone machine his or her own rearsets/footrests? How hard can it be? I need to look into that as well...
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Pretty hard, actually. (Or, should I say, time consuming.)
I don't know what it's like in your area, but, here, in San Francisco Bay Area, it won't happen unless you own machine shop.
Back in the early - mid 90s, when KOSMAN (Sandy Kosman, the founder and original owner of KOSMAN) was still in San Francisco, I saw him doing all kind of fabrication.
He would make things like SPONDON (style) banana swingarm.
But, even Sandy back then, wouldn't do custom. one off rear set.
(well, I guess he would have, if someone paid $5k or so in cash. I was a "small job" customer. Still, he was nice enough, he even lend me outrigger front sprocket once.)
Back then, when fitting CBR 600 F2 / F3 front end to EX500 was popular, a lot of people around here used machine shop for the project.
Most of the time, people (like myself) made the job small and paid cash, so that the machinist can slip them in between their "real" work.
When I had my (back then) beloved turbo GPz, Sandy was already moved to Windsor, but his top machinist J.P. opened his own machine shop in South San Francisco, and I used his service a lot.
He would make custom frame, custom swingarm, but wouldn't do rear set.
I ended up finding ZX-6R's foot control parts (a bit from here, a bit from there, true MAD MAX style), made a template of adapter plate, and asked him to machine it.
If you know exactly how much (and in which direction ) you want to move foot controls, you may be able to ask local machinist to fabricate the adapter.
Hint
Wayne Rainey's GPz 750 AMA Superbike racer
look closely, which part is fabricated, which parts are existing parts, how they simplified.
Something like this is doable, with the aid of local machinist.