How hard depends on how technical you are. It will be custom, you will need the motor, ecm, wiring harness, radiator possible, and lots of ability to handle custom fabrication. You will want the swing arm off of a 05 636 as well as they are braced and the extra 45+ rwhp from the 10R motor could possible flex you stock swing arm causing some suspension flex.
I ahve not see anyone do this yet, some of your other problems could be frame interferrence as the 636 frame may not be able to physically hold or mount the 10r motor.
I think it would be tons easier and cheaper to take a 05 10r and fit 04 636 fairings on it. Then all you have to do is come up with a single sided high mount exhaust and your good to go. Why mess up a perfectly good 04 636? Might not be as fun to do, but a lot less chance of you spending big money for something that never works right.
Oh, and i have no idea about the 05 10r's? Are those already single sided cans or are they dual? If they aren't dual then its even easier.
lol if you really wanna be cheap you could just put 636 stickers on the 10r and hope noone calls u out...they might but you never know hehe..
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