if you are looking to get production lines, you probably won't have a lot of options. odds are you could obtain tooling and plans for some of the conventional vehicles (especially the succession wars versions of common platforms) but as far as mechs go your odds are low of finding anything.
I figure the best bet would be to restore damaged infrastructure that's already there, but that may not be possible, and we're highly limited about how much we know about what Niops actually built. I believe only four designs were ever expressly named: the Nighthawk PA(L), the Black Knight, the Highlander and the Burke.
In terms of tooling availability, their best bet on the open market would probably be remaining Primitive/RetroTech lines from manufacturers who upgraded or are upgrading those production lines, or some sort of armed IndustrialMech. In a lot of ways, Niops is in a similar boat to New St. Andrews or Randis when they started manufacturing their own 'Mechs, though they might have the advantage of having some tooling still laying around.
My thoughts look at designs it should be easy to gain plans for as well as upgrades:
Sabre aerospace
Probably not a bad choice for an all-around interceptor, and it comes in a variety of tech bases, including Primitive, if that ends up being the best they could manage for a while.
Stinger/Wasp Mech
Wolverine/Griffin Mech for a trooper mech
Same tech base argument applies here, too.
That said, if they recover enough for Intro tech, though, I wonder if some version of the Phoenix Hawk to fill both roles might be better. We know that, at one time, Niops did produce 270-rating engines, too. Alternatively, the Chameleon could maybe serve in a similar role, and could also function as a trainer 'Mech in their academy, and also uses a 300-rating engine, like their old Black Knights used to.
Failing that...there's always the Quasit. ;D
Goblin tank and look at Daryk designs for a family of units
Either Gray Death BA or Cavalier BA suits
Goblins are a solid choice. As for the BA, I'd almost rather see them keep the Nighthawks in production, if only for continuity with pre-Jihad Niops.