Please check my parsing of the troublesome line.
Troublesome line marked ** ... **
hic mortis durae casum tuaque optima facta
**si qua fidem tanto est operi latura vetustas**
non equidem nec te, iuvenis memorande, silebo.
Aeneid, Book X, ll. 791-793; Loeb Classical.
my translation of marked line:
If by-any-way belief in-such-great deeds antiquity is to-be-about-to-be-bearing
My parsing:
On my reading, "fidem" is the direct object of the future active participle "latura".
"latura", feminine singular nominative, modifies the noun "vetustas", as a predicate adjective.
"vetustas" is the subject of "est".