I've built more hogs out of aftermarket parts then Ihave with original parts.
back in the 60s & 70s we'd scrounge alltehparts we could and 'build a bike' bought three running bikes in my life, built too many to count for myself and others.
Buy (or build) a frame, get a set of cases, by flywheels (stroked for sure) custom forged piston blanks, bough al my bearngs loose in a bag or box. Even built a set of cylinders out of weldment of crome moly tube and high stregnth plate good for drags only no fins.
you take a cuttin' torch, grinder and tig torch to a frame and then go from there. One time it took six months to locate the parts for a very stock appearing 53 panhead. Fuckin built my rep big-time there. Weren't any aftermarket parts back then. Finding that '36 seamless oil bag and VL springer was an acheivement that had the entire biker community talkin' tha bike was a collection of parts from '51, '53 '36 and alot of '75 home made shit.
i think that building parts with mill, lathe, blanchard grinder and hand tools qualifies as building the bike. Everything is just parts, until you make it whole. it's what builders do. Hell, the Space Shuttle is built with (mostly) off-the-shelf parts. Any engineer worth his salt will have a stack o' catalogs higher than he is tall when he sits down to design, bikes ain't no different.