Maybe it's good to start of with what we're actually taqlking about.
Chapter XVI of Beowulf is often referred to as "the Lay of Finnsburgh"
It is when a bard sings an old glorious tale. So here Beowulf incorporates an older text. However, it is assumed to be a well known text and the bard only cites the best part of it; i.e. the blood and action stuff.

Only 48 lines have been preserved of the original Saga of Finn Folcwalding, as quoted below (including English translation).
This is generally referrred to as: "The Finn Fragment" to distinct it from the part incorporated in Beowulf.

...nas byrnað?"
Hnæf hleoþrode ða, heaþogeong cyning:
"Ne ðis ne dagað eastan, ne her draca ne fleogeð,
ne her ðisse healle hornas ne byrnað.
5 Ac her forþ berað; fugelas singað,
gylleð græghama, guðwudu hlynneð,
scyld scefte oncwyð. Nu scyneð þes mona
waðol under wolcnum. Nu arisað weadæda
ðe ðisne folces nið fremman willað.
10 Ac onwacnigeað nu, wigend mine,
habbað eowre linda, hicgeaþ on ellen,
winnað on orde, wesað onmode!"
ða aras mænig goldhladen ðegn, gyrde hine his swurde.
ða to dura eodon drihtlice cempan,
15 Sigeferð and Eaha, hyra sword getugon,
and æt oþrum durum Ordlaf and Guþlaf,
and Hengest sylf hwearf him on laste.
ða gyt Garulf Guðere styrde
ðæt he swa freolic feorh forman siþe
20 to ðære healle durum hyrsta ne bære,
nu hyt niþa heard anyman wolde,
ac he frægn ofer eal undearninga,
deormod hæleþ, hwa ða duru heolde.
"Sigeferþ is min nama," cweþ he, "ic eom Secgena leod,
25 wreccea wide cuð; fæla ic weana gebad,
heardra hilda. ðe is gyt her witod
swæþer ðu sylf to me secean wylle."
ða wæs on healle wælslihta gehlyn;
sceolde cellod bord cenum on handa,
30 banhelm berstan (buruhðelu dynede),
oð æt ðære guðe Garulf gecrang,
ealra ærest eorðbuendra,
Guðlafes sunu, ymbe hyne godra fæla,
hwearflicra hræw. Hræfen wandrode,
35 sweart and sealobrun. Swurdleoma stod,
swylce eal Finnsburuh fyrenu wære.
Ne gefrægn ic næfre wurþlicor æt wera hilde
sixtig sigebeorna sel gebæran,
ne nefre swetne medo sel forgyldan
40 ðonne Hnæfe guldan his hægstealdas.
Hig fuhton fif dagas, swa hyra nan ne feol
drihtgesiða, ac hig ða duru heoldon.
ða gewat him wund hæleð on wæg gangan,
sæde þæt his byrne abrocen wære,
45 heresceorp unhror, and eac wæs his helm ðyrel.
ða hine sona frægn folces hyrde,
hu ða wigend hyra wunda genæson,
oððe hwæþer ðæra hyssa . . .


TRANSLATION
..... ‘the gables are not burning.’
Then the king, a novice in battle, said:
‘This is not dawn from the east, no dragon
flies here, the gables of the hall are not burning,
but men are making an attack. Birds of battle screech,
the grey wolf howls, spears rattle,
shield answers shaft. The wandering moon gleams
under the clouds; evil deeds will now
be done, bringing grief to this people.
But rouse yourself now, my warriors!
Grasp your shields, steel yourselves,
fight at the front and be brave!’
Then many a thegn, laden in gold, buckled his sword-belt.
Then the stout warriors, Sigeferth and Eaha,
went to one door and unsheathed their swords;
Ordlaf and Guthlaf went to guard the other,
and Hengest himself followed in their footsteps.
When he saw this, Guthere said to Garulf
that he would be unwise to go to the hall doors
in the first rush, risking his precious life,
for fearless Sigeferth was set upon his death.
But that daring man drowned the other voices
and demanded openly who held the door.
‘I am Sigeferth, a prince of the Secgan
and a well-known warrior; I’ve braved many trials,
tough combats. Even now it is decreed
for you what you can expect of me here.’
Then the din of battle broke out in the hall;
the hollow shield called for men’s hands,
helmets burst; the hall floor boomed.
Then Garulf, son of Guthlaf, gave his life
in the fight, first of all the warriors
living in that land, and many heroes fell around him,
the corpses of brave men. The raven wheeled,
dusky, dark brown. The gleaming swords so shone
it seemed as if all Finnesburh were in flames.
I have never heard of sixty warriors
who bore themselves more bravely in the fight
and never did retainers better repay
glowing mead than those men repaid Hnæf.
They fought for five days and not one of the followers
fell, but they held the doors firmly.
Then Guthere withdrew, a wounded man;
he said that his armour was almost useless,
his corselet broken, his helmet burst open.
The guardian of those people asked him at once
how well the warriors had survived their wounds
or which of the young men ....