What did you do with whatever else you've got?

[nick crisp]

a Kawasaki Z650 tailpiece and seat

Nice, but the best looking bit is the Z650 tail unit with 2/4 seat. But then, I'm a dyed-in-the-wool Z650 fan 
The pipes look good there too.
If i had the money to be buying older bikes, I'd be wanting to do similar mods. Mostly contemporary kind of things from their day, plus handling, and maybe a little tuning.
My ultimate project would have been a Z650 with 750 barrels taken out to 810. A chap who used to work at a local bike dealer had this done to his, and it's something that I've wanted to do for donkey's years. Improve handling (I've seen one with RGV front and rear ends) and chassis strength, but retaining the standard look as far as bodywork/lines go. Alas, I never had either the money or skill to do such a thing.