It's 本歌取り "adaptation of a famous poem". The original poem is in 万葉集.
今日よりは返り見なくて大君の醜の御楯と出で立つ我れは
大君のしこの御楯 means Emperor's shield. しこ(醜)is a humble expression here.
大君のしこの御楯と出で立ちし modifies 君.
It's 君
が武運, not 君の武運. (The meaning is the same, though.)
Your understanding of the last half of the poem is correct, but Kamata is the one who wrote this poem, not 君.
The literal translation is;
I pray from the bottom of my heart for your good luck, who left home/country as the Emperor's shield.
There is a red square stamp of 官幣大社諏訪神社 Kanpei-taisha Suwa-jinja (the present 諏訪大社 Suwa-taisha) on the upper right.