OK, hear me out here...
Niops had their own little civil war, then a short war against the Marians, then a short brutal war against the Word of Blake, then took their time getting their crap together over the next five or six decades, right? But, hey, they're all united now, and they're working with Interstellar Expeditions, and spending more time in space than ever before. They may have lost their workshops, but they've got access to a variety of technical data, and while they'll probably build out plenty of conventional forces, having a ready space force and BattleMech force are both rather important.
Three relatively ubiquitous intro-tech BattleMech designs that could still see a great deal of utility in the 3140s are the following:
RFL-series Rifleman: The RFL-1N was reintroduced during the Jihad era, and while it's mainly pirates using it during the 3130s and later per the MUL, it should be possible to get ahold of that tooling. The RFL-3N, meanwhile, is still out there, and even on the Periphery General list per the MUL, so getting a production license form Kallon may be possible.
ARC-series Archer: The ARC-1A primitive version got reintroduced during the Jihad, too, and while it appears to have fallen off in terms of production, it may be possible to get the tooling. The ARC-2R Archer is also out there still, and on the MUL for the Periphery General. It might be possible to get tooling for that, too.
WHM-series Warhammer: No primitive version, but the WHM-6R is common as dirt, and still on the Periphery MUL list. You may be able to start building that, too, if you can get a license and tooling.
For a scout design, Niops could standardize on a design that could doble both as a scout 'Mech and an aerospace fighter, and go with one of the following:
WSP-1## series Wasp LAM
STG-A#series Stinger LAM
PHX-HK# series Phoenix Hawk LAM
Since these would be fielded by a united Niops Association, and quite often in space, either supporting the asteroid mining or otherwise, you could even shuffle them into a new division of the Niops Association Militia, the United Niops Space Army, or UN Spacy for short. ;D