For the record, I bought the bike for this purpose. It is a '72 frame with a '77 engine, and god-knows-what year wiring. Has an electric start with none of the electronics to match. Not sure of the fork vintage, but I got two sets with it. Seat needs recovering at least, engine has been spray-bombed black... you get the idea.
The other one I bought (blue in the pics) is a complete 1971 XS1B which is much more of a candidate for a restoration. It's the last of only a 2 year run of the original XS650 design, designated as XS1 and XS1B. It is going to my brother, who recently completed a frame-up resoration of his BMW R75/7.
I have a 1975 BMW R75/6 which is a true Survivor. Perfectly original from the factory. Never repainted, never restored, heck, not sure if it's ever been cleaned. Lots of nice patina on the aluminum from 36,000 miles. Original cables, carb boots, grips, footpegs, plug wires etc.
My CB550F went out the door of my garage in perfect original condition - NOS seat, perfect 1975 Sunrise Orange bodywork, original exhaust, NOS switch pods left and right - I spent WAY too much money making it original buying loads of NOS parts from my Honda dealer, right down to sidecover grommets, tank mounting rubber etc. even though they didn't need to be replaced and were hidden from sight.
I think I've earned the right (not that I need to earn it) to hack up whatever old frame I feel like. If modified bikes are not your cup of tea, then drink your coffee and enjoy your phillps head screws, weak ignitions, poor suspensions etc. The bloody swingarm bushings on the XS650 are PLASTIC - should I leave them that way for the sake of keeping it original? How far does it go, or is it only about appearance for you?
Heaven forbid anyone do the fool thing of putting a Triumph engine in a Norton featherbed frame - the Queen would have a fit!