The lock is held in place by a small roll pin and can't be removed intact without being turned.
Since the lock is difficult to access without removing the forks and or triple tree I would try picking the lock first. They are simple locks, pretty easy to pick.
After that, drilling is an option but again it's difficult to access because of the location.
Also, I assume you only have an ignition key because the ignition switch was changed.
Do you have a key for the seat lock?
The original seat lock and fork lock would have had the same numbers.
On the later seat locks the number is stamped on the back of the lock.