Here is my translation

Alan

(Önundur leiðir Þóri til fyrirsátursmannanna. 
(Önundr leads Þórir to the-ambushers.

Þar tekst mikill bardagi. 
There takes-place a great battle

Þórir fellir marga menn en loks eru höggnar af honum báðar hendurnar.)
Þórir fells (kills) many persons (men) but finally both (the) his arms are cut off.

Þórir hljóp þá að gljúfrunum og sló stúfunum við hellu mikilli svo að hún hrökk til. 
Þórir ran then to the-rocky sides-of-the-ravine and struck with the-stumps (of his arms) against a large flat-slab-of-rock (hella, f) so that it drew-back.

Þá sáu þeir að kisturnar er Valshellisgull var í voru þar undir. 
Then they saw that the-chests in which was (ie containing) (the) Valshellir (Valr´s-Cave) Gold were there-under.

Þórir stakk handarstúfunum sínum í hvorn kistuhring og steyptist fram af í gljúfrin í fljótri svipan og sáu þeir hann eigi framar.
Þórir thrust his arm-stumps into each chest-ring and tumbled forward off (ie away) into the-rocky-sides-of-the-ravine in a fleeting (seems to me fem dat sg ending rather than comparative) moment (ie in the blink of an eye) and they saw him not more.

(Loks segir frá afdrifum félaga Þóris og sona.)
(Finally(it) says about (the) fate of Þórir’s comrades and (of his) sons.)

Guðmundur bjó á Þórisstöðum til elli og er mart manna frá honum komið. 
Guðmundr dwelled at Þórisstaðir (Þórir’s-Steads) until old-age and a great (number) of persons (men) (note: gen plural) is/are (has/have) come (descended) from him.

Atli bjó í Gröf og voru þeir bræður mikilmenni. 
Atli dwelled in Gröf and those brothers were powerful-men.

Vöflu-Gunnar bjó á Gunnarsstöðum en Grímur í Múla langa hríð en Þorsteinn Kinnarson á Kinnarstöðum. 
Vöflu-Gunarr dwelled at Gunnarsstaðir (Gunnarr’s-Steads) but (and) Grímr in Múli (Mull) for a long while but (and) Þorsteinn Kinn’s (Cheek’s)-son at Kinnarstaðir (Kinn’s (Cheek’s)-Steads).

Þar heita nú Leikvellir er Þórir féll. 
There (ie that place) now is-called Leikvellir (Play-grounds) where Þórir fell (in battle)

En um hann veit engi fremur að segja eftir það er hér var sagt.
But (And) about him no-one knows further to say beyond that which here was (ie has been) said.

Þórir hefir verið einn með hraustustu mönnum hér á landi. 
Þórir has been one among (ie of) (the) most-valiant persons (men) here in (the) land

Sá hann og flest fyrir fram og var vitur maður og engi ofsamaður allt þar til er lund hans spilltist og hefir hann þá goldið þess síðasta drykks sem hann drakk af kálki Ragnars eins og Ragnar fyrir sagði honum og hér var áður sagt.
He saw also most before forward (ie into the future?) and was a wise man and no overbearing-person (man) entirely until his mood was-damaged (soured) and he has then paid for (suffered on account of, pp of gjalda) that last (ie the one too many) drink which he drank from Ragnarr’s goblet (chalice) just as (eins og) Ragnarr predicted (lit: said before, segja fyrir) to him and here was (ie has been) already said.