Ahh ! Inside the amateur 70cms band. In which case you may be lucky
if it uses a relay to switch the antenna. You will need to break out
the TX line to feed a linear amp. Just watch the power rating of the
relay. As far as the receiver is concerned, you need to improve the
noise margin without adding too much gain. 5 or 6 Db should be more
than enough. Noise figures for a front end should be around 1.5Db or
better. A Yagi antenna will provide both gain and directivity more
cheaply than either of the other solutions though !
Assuming that the "base" station is fixed, a large biconical antenna may
give you the horizontally omnidirectional gain you need. The thing would
probably be 6 foot diameter and 2 foot tall. This should give you between
8 to 12 dB gain. Flatland assumed.