A tunable high-Q loop antenna, such as:

https://www.amazon.com/Kaito-Tunable-Passive-Antenna-Panasonic/dp/B001KC579Q/

placed near your small AM radio can inductively couple into the internal ferrite antenna or RF front-end, and add to their gain.

Here's an example of one built around a crate than you can put your radio inside:

https://swling.com/blog/2017/06/how-to-build-a-milk-crate-am-broadcast-loop-antenna/

Basically, you construct a large (larger than your radio), multi-turn air-core inductor, and tune to stations in the AM band using an air variable capacitor. The much higher Q of the large air-core inductor and air variable capacitor can also help separate adjacent frequency AM stations.

A tunable high-Q loop antenna, such as:

https://www.amazon.com/Kaito-Tunable-Passive-Antenna-Panasonic/dp/B001KC579Q/

placed near your small AM radio can inductively couple into the internal ferrite antenna or RF front-end, and add to their gain.

Here's an example of one built around a crate than you can put your radio inside:

https://swling.com/blog/2017/06/how-to-build-a-milk-crate-am-broadcast-loop-antenna/

Basically, you construct a large (larger than your radio), multi-turn air-core inductor, and tune to stations in the AM band using an air variable capacitor. The much higher Q of the large air-core inductor and air variable capacitor can also help separate adjacent frequency AM stations better than the tiny dial on your pocket radio.

If you just want to add a long wire antenna, and have a large enough yard or field to put it in, around 250 to 750 feet of wire suspended above ground might be suitable for improving AM broadcast band reception.