The simple answer is "don't use the passive voice in such constructions."
If it is an exam question then you can form the passive in the main clause:
If you can't get to the station on time, the tickets can't be got.
That is already horrid, but correct.
You could put the passive in the condition:
If the station can't be got to on time, you can't get the tickets.
Also horrid.
You can do both:
If the station can't be got to on time, the tickets can't be got.
Yuck.
The condition can be put after the main clause, either passive or active in either clause
The tickets can't be got if you can't get to the station on time
The tickets can't be got if the station can't be got to on time.
You can't get the tickets if the station can't be got to on time.
All very horrid. All correct.
The only thing that you can't do is make a passive of the modal. You can't grammatically say "The tickets aren't could get...".
In particular, using the passive doesn't mention who is getting the tickets, and so implies "by anybody". As mentioned in a comment: "it's grammatical but odd to mix the [voices] because it changes the meaning. For example, 'If you can't reach the station on time, the tickets can't be bought.' is confusing, because who is buying the tickets? You've already identified yourself as the subject, so why use the passive unless you mean that someone else is going to buy them" (— Andrew)