The fork legs could be bent
Triples could be bent
Steering head could be twisted slightly
Steering head could be "straight" but pushed to one side
I would start by removing the forks and seeing if the legs or triples are bent/twisted.
Check rear wheel alignment as the distance from center of the swingarm spindle to the center of the rear axle.
Then I would set the bike up so that the rear wheel is exactly vertical. Use common house shims under the stand until it's perfect.
Then try to align the front wheel but do not adjust the rear wheel at this point. The front wheel will be, perfect OR it will be off to one side OR the front wheel will not be vertical.
If the front wheel is off to one side and not vertical, the steering head is tweaked and will need to be straightened. That requires an expensive jig, but it can be done manually if you have 4-6 strong mates and some long lengths of steel tube. Several tubes are placed through the frame counter to the way that the frame needs to be bent, and strong dudes hang onto them. Another 2 dudes place another steel tube through the steering head in a way that allows them to twist it back into place. They will need to twist way beyond the right place because it will spring back.
While you are doing all that you have to check that nothing cracks or gets bent out of place too far. I would not recommend this approach unless you get it inspected after by a trained engineer who can determine if the frame was damaged. Any damage to a frame is not trivial so repair it with caution.
But check it all before you resort to that.