If you didn't pull the buckets off at all, then possibly a bucket moved if you tilted the head while carrying it. In that case, there is a chance the shim slipped out of position. But it would have ran like total crap since at least one valve would have been open all the time.
Well, assess the damage, and maybe take a quick look on ebay or Craigslist for motors or parts bikes. Usually a good set of gaskets will be more expensive than a used motor. But a used motor may need work, so it's a toss up.
You can use C model motors, A models up to 83, I think the last year for the A model had a different intake. You could also use an 81 gpz motor. Other KZ550's will use different carbs and have different intake port sizes from yours.
