1/4 scale tailpipes? = Muffler shops. Many imports still come out with pretty small exhausts. 1-1/4" isn't too hard to find from my knowledge. They could even bend it for ya. Slash the ends and then get it nickle plated/chromed.
1/4 scale motorcycle seat? Make it. Seat shapes are dictated by their seat pans. You cut the shape, put some foam on it, and then cover it. Any moto-shop has this stuff. And you can have embroidering shops do a design if you want. Walking down any mall makes me trip across an island trying to sell custom embroiders.
1/4 scale fenders? Hmmm...that depend on where you're going to put it. If I can picture this right, you don't want a full fender. Perhaps a 1/4 fender that looks like a hoodie popping out the front. That would take a piece of sheetmetal cut out in an arch, then simple bending it around a large pipe (Pipes are everywhere if you start looking for them. Usually pipe-bullards are located around parking lots). Trim and debur...done.
I'm not sure if the Northwood 3.4EE is that awesome in 478 form. It may be an oversight, but the AA8 (LOOOOVE it) is 775, so...This 3.4EE
may be the better choice
Going on the OCC idea, I think it'd be cool to get the tallest finned CPU HS you can find, but in aluminum...and for the 775. Then, anodize it black and sand the edges down to expose the aluminum on the "fins". Hide the fan with a laser cut chrome grill and there...instant Harley cylinder head.
But you know what? What ever happened to Easy Rider?? OCC may be "fadish" as who knows if it'll be around for another year. Easy Rider, Scream'n Eagle (Harley's performance line), or even more wicked...Arlen Ness would be longer lasting. If someone would say, "Hey! That gear shifter on the side that turns it on is an Arlen Ness", then the point is made and everyone can be happy.