Extend the power

There is really no easy way to do this and even if you do there are other considerations to take into account. You would have to open up the back of the radio and do some modifications. See where the telescopic antenna is soldered to the PCB. Unsolder it and replace it with a short coaxial cable with an RF connector on the other end. The center of the coaxial cable should go to where the antenna was connected and the shield should go to the ground plane on the PCB. Use an extenal antenna and connect it to the RF connector that you just attached to your radio.

As you can see this is alot of work and if the radio was beautiful to start with it certainly won't be now with the wire hanging out the back of it.

Even if you do this you may make matters worse because with the higher gain antenna the input levels will be a lot higher and you may overload the radio's front end causing spurious signals that mask any weak signals that may be present.

After all this I would say get another radio with an external antenna connection and use an external high gain antenna.