I have had an ACL reconstruction 8 years ago - never had problem with the knee (till I made an unplanned summersault again last year skiing
) and never had any problems getting a class I medical.
As long as the knee is stable, doesn't give you any problems, there is no reason why you shouldn't be able to get the medical - before or after the reconstruction.
@ Mohit_C: recovery time varies a lot - depending on your overall fitness and the method used for reconstruction.
I was allowed to put on full weight the day after surgery - and released for about anything 5 weeks after reconstruction (wasn't allowed to do flips on the trampolin for 6 months though
)