I am interested in this mc21 but it's got over 30,000kms with unknown history. he doesn't know when the last rebuild was. besides needing a new topend, should I expect the crank to be gone at this point?

It's possible of course, though not likely if the thing has been looked after.

What does the general condition of the bike tell you? Is it clean with no alloy corrosion? Are the footpegs ground down? Is it standard or restricted? How much wear left on the brake pads, tyres, chain and sprokets? Are there dings on the wheels from race tyre changes? Are the fairing brackets bent? Is the tank chipped? Is the bodywork immaculate, but the rest looking tired? Could have been raced and road bodywork fitted back on later. Any of the bolts rounded off? Are there paddock stand bobbins fitted to the swingarm? Any black gunge on the engine, or in the bottom of the fairing? Are the lockstops drilled?

Basically, if the bike is gleaming, sounds nice, and needs no work to ride it right now, it's probably ok. If it needs new tyres, brakes, etc etc then walk away as it's gonna cost you before you even factor in the rebuild possibilty.

Oh, one last thing. Do a compression check. Anything over 110psi is good and the motor is fine.
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball