Can you get a girl's interest back?

The situation is very specific. This girl was and is on a mission for the LDS Church. During the mission, dating is prohibited and she doesn't have a private number.

She looked very interested in me, but then I noticed it waned a bit when they assigned her a new companion. The problem is that her companion probably liked me too, even though I'd only met her once months earlier. She remembered me exactly and said we could finally spend more time together (the first time asked me a lot of stuff like if I had a car and a house, if I made good money, etc.)

I don't live in the US, there are few members here and it happened to me twice before that two people who were friends liked me and at a certain point I ended up being ignored by both of them to maintain their friendship.

This girl is the only one who's really interested in me. The fact that she's still on her mission an that is a huge obstacle. I'd be patient and wait. But in your opinion, given the delicate situation, if I were to see her again, how could I let her know I like her and want to date her?

She's in another city now, but I know she'll be back in my area sooner or later.

I don't want her to lose her enthusiasm. Because I've experienced with others that not acting at the right time has made them lose interest.