> og mundi eigi út leitað viðfanga ef gnógt væri inni
> and (they) would not reach for/seek provision (?) if it were within

Yes, that's more or less the idea: "and supplies wouldn't be sought
for if there were enough inside". 'gnógt' "enough" (neut.nom.sg. of
'gnógr' = 'nógr'). 'vi-fanga' is the gen. of 'viðföng' "provisions,
supplies" (neut.pl.). 'mundi eigi út leitað viðfanga' is passive.
When a verb takes a dative or genitive object in the active voice, the
passive has to be formed using an impersonal construction (like you
get in German), lit. "it wouldn't be sought for supplies", where we
would have to say in English "supplies wouldn't be sought for". But
it's often more natural, as you've done, to translate examples like
this with the active: "and he wouldn't be looking for [fresh] supplies
if he had enough inside".