You won't be able to use the ACARS dipole, it's for a totally different frequency.
As for the connector problem, it is an SMA connector. I don't know if Maplin stock them but it should be covered by Airnav's warranty. Are you saying that the pin in the connector has come out? You mentioned the SMA connector on the box as well, that is much less likely to be affected but it is possible. So which is it?
I don't know what sort of SMA is used, there are different versions. If the pin is a soldered part then it is possible that the joint has failed. There are also crimped parts available, in that case the crimping pressure could be too low and hence the pin slides off the inner conductor.
I'd say it's a warranty job myself.