One way to search for other KAPLANs from Motol is to go to www.stevemorse.org and search the Ellis Island Database for surname KAPLAN and town name Motol. I got 2 hits:
Kaplan, Leie, arriving 1905
Kaplan, Piukus, arriving 1906
Looking at the manifests at the above site should help you figure out if they are connected to your KAPLANS in Chicago.
Also, at www.familysearch.org a search for surname KAPLAN and birthplace Motol got one hit:
The WW1 draft card of Morris KAPLAN, born Motol, living in Chicago. The draft card, which can be viewed at the above site, will give your more data on Morris.
Hope this helps,