Well, you can always try to find something trough the digital internet archive

Here are some archives of the club's official site, some as old as 2004.

https://web.archive.org/web/*/marsonija

Note that pages that old and somewhat obscure are heavily fragmented and mostly contain only partial achival data.

Also, here is a link to the digital archive of "Glas Podravine" using the search word "Mandžukić"

http://library.foi.hr/glas/rezultat.aspx?g=&u=mand%9Euki%E6

Unfortunately it seems that the only reports mentioning his name there are related to the games played against Koprivnica, a club from Podravina.

Here is an example

"KOPRIVNICA PROPUSTILA LIGU ZA PRVAKA

Koprivnica -Marsonia 0:3

KOPRIVNICA -Gradski stadion. Gledatelja: 200.

Sudac: Rapić (Vmkvci) 7.

Strijelci: 0:1 -Praiz(3), 0:2-Prgomet (73), 0:3 -Mandžukić (74).

KOPRIVNICA: Sokač 5, Volf 5, Kuhar 5, Balaško 5.5, Međimorec -(od 25. Crnila 5), Tukser 5 (od 46. Horvat 5), Ogrizović 5, Purić 5.5, Serda-rušić 5 (od 54. Maljak 5), Omrčen-Čeko 5.5, Galinec 5.

Trener: Zvonimir Kelek.

MARSONIA: Vidaković 6.5, Bogdanović 6, Božić 6, Brkanac 6, Mus 6.5, Kruljac 6, Dujak 6 (od 76. Zovko -), Praiz 7, Alić 7, Mandžukić 7.5 (od 77. Mikulić -), Kljaić 6 (od 60. Prgomet 7).

Trener: Josip Markovinović.

Nakon izgubljene utakmice s Marsorujom nogometaši Koprivnice najvjerojatnije su se oprostili od nastupa u Ligi za prvaka. Samo pobjeda kod vodeće Cibalije (ponedjeljak) vratila bi Koprivničance u prvu polovicu ljestvice"

But there might be a similar archive on the internet. "Glas Slavonije" would be the proper paper to study in this case since Marsonija is a club from Slavonia, but as far as I can tell, they don't have a digital archive.

You might also try to contact the journalists from Utakmicu po utakmicu on facebook or instagram. They might have access to better source material and are receptive to fan communication.

Here is the facebook page:

https://www.facebook.com/Utakmicu-po-utakmicu-101695938352637

Instagram profile:

https://www.instagram.com/utakmicu_po_utakmicu/

Also, there's a library catalogue containing "Sportske novosti" which roughly translates to "Sports News" a paper dedicated to sports journalism. As far as I can tell, there are no online digital archives, but you could try contacting a library that has copies of the paper published a day after those games were played. Who knows, someone might be willing to assist with your research. They might be more receptive if you intriduced yourself as a journalist or a student working on his thesis.

https://katalog.kgz.hr/pagesResults/bibliografskiZapis.aspx?&currentPage=1&searchById=20&sort=0&spid0=20&spv0=sportske+novosti&selectedId=35003033#lokacije

You just select the year, month and day, and you'll see which library has a copy of it. You can then contact them via e-mail.

You can also try to contact someone from SBTV

https://www.youtube.com/watch?v=kA1GAyS8_MY