Karges was familiar with the compositions of Sweelinck through Andreas Düben (1597 - 1662). Düben was a pupil of Sweelinck in Amsterdam from 1614 to 1620. And when Düben became organist of the German church in Stockholm in 1625, Wilhelm Karges was his assistent for a short while. This explains the familiarity of Karges with thw works of Sweelinck and his pupils.
Karges must have had access to written source material of the music of Sweelinck and his pupils. Yet in manuscript Am.B 340 he does not simply copy this music, he invariably adapts it. Most of Sweeinck's music is manualiter, Karges adapts it to pedaliter (even with short manuals, the distance between bass voice and tenor is sometimes too large to be able to be played with hands alone), and he makes it longer or shorter to fit his needs. Those needs were probably that of practical use in church service, where he needed music of a certain length to play.
Karges' Fantasia in D, written on folio 24 of the manuscript, is based on Sweelinck's Fantasia SwWV 260. Karges mixes elements from that Fantasia into a rather nice, not too long, Echo Fantasia. The manual changes are clearly indicated in the manuscript.
Score available here: http://partitura.org/index.php/wilhelm-karges-jan-pieterszoon-sweelick-fantasia-in-d-after-swwv-260