The known Swedish sources for the encounter at Systerbäck on 9 July 1703 are few. Most extensive is Relation von der Action, so den 9 Julii 1703 bey Sösterbeck zwischen der unter meinem Commendo stehenden under feindtlichen armee vorgefallen, printed by Yrjö Koskinen in 1865. This account was sent by Cronhjort to "Defensionskommissionen" on 14 July and is preserved in the committee's archive.
A couple previously overlooked Systerbäck sources can be found in LVVA, fond 7349, op. 1, vol. 358 as attachments to a letter from Lt. General Georg Johan Maydell to Governor Frölich in Riga. Maydell had at that point not yet reached the army in Finland, but as a long time commander of a Finnish infantry regiment he undoubtedly had many contacts in Cronhjort's army. In the letter Maydell writes that the losses luckily hadn't been as bad as the rumours had been suggesting despite the fact that the action had started at 6 in the morning and continued until 2 p.m. The Swedes had lost 250 killed and 220 wounded, but the Russian losses were twice as high, Maydell claimed (according to Cronhjort's account there had been 203 killed and 184 wounded on the Swedish side).
Maydell's sources seems to have been two letters, one from captain Maydell and one from captain Paykull (summaries attached to the letter to Frölich).
Riksarkivet, ÄK 243, vol. 107 (Letters to Defensionskommissionen from Cronhjort 1703)
LVVA, fond 7349, op. 1, vol. 258 (Miscellaneous letters 1693-1703)
Handlingar till upplysande af Finlands öden under det Stora nordiska kriget / utg. af Yrjö Koskinen. - Helsinki, 1865