I'm thinking that one of the ethnic instruments should sound pretty close to a mandolin.
My Yamaha PSR-E443 has a mandolin sound, so I tried it, then tried the ethnic/world instruments to see how they compare. I think the rabab sounds pretty close. I haven't tried it on my nephew's WK-110 yet, but on the CTK-860IN the rabab is tone number 452. The banjo also sounds somewhat like the mandolin-- on the CTK-860IN the banjo would be tone numbers 448 or 561. There's also a muted banjo, tone number 449 on the CTK-860IN; but on my PSR-E443 the muted banjo doesn't sound as much like the mandolin.