What model bike are those parts off of?... they can't be cheap.
Aside from the obvious questions of whether or not that works handling wise on the CBR, or what it takes to fit it up properly to the bikes frame. the big question is how long do you plan on keeping the bike? It's one thing to do a big modification for your own satisfaction, etc... but, if you think you may move on to another bike in a couple years (perhaps a sport bike with a single sided swing arm right from the factory), you can't expect to get the money you spent back at resale. Safe to say, a lot of used bike buyers would take one look at such a mod and say "no thanks".
Something else to consider, is your insurance coverage... it's certainly possible that if you did a major modification like that to the bike, and at some point in the future found yourself in an accident, that your insurance company could deny any and all claims, simply based on the fact that the bike was altered from the original factory build/design in a major way. And we all know that insurance companies will always look for any possible reason not to pay out on a claim.
The pics are from a VFR800 (I only used those screenshots because the quality of the pics is really high and illustrate the swingarm/shock/rotor/sprocket/wheel assembly interaction clearly)
If I do this, it will be done using the SSS from a JDM Honda VFR400 or one of the SSS JDM VF/VT R250s. The key is to locate one with the proper swing arm pivot dimensions. From there I can machine shock linkages, spacers, etc...that's the easy part...
It's not a difficult mod to perform. I have this mod before, though not to my own bike. I worked with my friend to do this mod to a mid-90s CBR600, utilizing the SSS from a Hawk GT 650. In his case, we had to machine a spacer. The Shock lined up perfectly in that case and, as long as I get the right JDM SSS, the shock should line up as well.
Price-wise, the pics I posted were from an eBay auction/buy it now for $699 for the whole assembly. There are also ones not quite as clean looking for $599, $479, $359 on eBay, but I can't link to them while at work
That's less than the cost of a higher end Garmin Montana GPS, necessary wiring for powering it, and RAM Mounts. It's also less than the cost of an exhaust system.
My friend's CBR's ride characteristics were no different because the wheelbase remained unchanged. Weight wasn't changed appreciably, and weight distribution remained the same. I see no reason to believe the ride characteristics would change on the CB300 if wheelbase and weight distribution remain the same.
None of the mods we made to my friend's bike were made to the bike...the only parts to be modded were the ones he purchased...meaning the bike can be returned to stock at any time.
Cool, sounds like you have the experience and skills required for the job. I'd imagine the two criticals would be alignment of the shock and the sprocket hub. The rest not so difficult to sort. Your only running a flexible line for rear braking for example. Not like the old drum brake days where you had linkages to deal with.
If you decide to go ahead keep us up to date. Would be a cool project.