I think it could be done, but I can tell you two things for absolute certain... (A) it will end up WAY more expensive than an MC16 or MC18, and probably even an MC21/28, and (B) it will shake itself to bits!
A CR500 transplant has been attempted a couple of times over the years, and it just doesn't work. Not saying that the implementations I've seen couldn't be more refined, but the nature of the motor just doesn't really lend itself to a "road racer" configuration. It's rather harsh, vibrates a lot, with a low peak RPM.
The exhaust would take a lot of work to get something even remotely suitable "hidden" within the bodywork. Again, something that could probably be done if you have the required skills, but definitely not easy, and it's not as simple as changing the "shape" of the exhaust to overcome the characteristics of the MX motor, to make it more akin a road racer. You have to look at port heights/timing, port shape, combustion chamber design, compression ratio, ignition timing. Remember too, the CR is "piston ported", with a cruder RC Valve, that's much more off/on in its power delivery.
The CR motor has solid engine mounts, further adding to its harshness in a street bike chassis.
Bear in mind that a CR500 donor bike is itself becomming more and more expensive too. (You're better off restoring the CR and selling it, than wrecking one for the motor, in my personal opinion.)
Unless you already have one lined up or in storage, I think you will also find an MC19, certainly an MC22, is rapidly appreciating in value too. I think it's going to get very expensive, very quickly, but good luck if you take the build on, and we'd love to see how you get on.
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Andy.
NSR-WORLD.COM
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