That's like saying why don't you just buy your new bike from the parts catalog 1 part at a time. Then you don't have to change anything. Am I missing something here? Not changing the crank. Just having it welded. As for the cams, I don't like the 266E that come stock on the 120R.
Actually there's a significant difference between building an engine for an existing bike versus building a bike from scratch. But you knew that.... Since you suggested it, however, it's been done.
http://www.cvoharley.com/smf/index.php?topic=43691.0It was a fun project too. And it was cheaper than buying a bike, gutting it and rebuilding it as I might have liked. None of this responds to the question you didn't answer though. Was just curious if you'd considered starting with a set of the available crankcases that are options and building your motor from the components you wanted as opposed to breaking down an existing new motor and having the redundancies? Wondered if there was a cost advantage (either way) and/or how it might have impacted the build options.
Didn't seem an untoward question....